docs: guide: Add build system information

A detailed overview of Zephyr's build system. This is a
thorough view of the low level build process starting from CMake and
using Make as the build system tool. Things missing here that will be
further documented:

- west
- external modules/libraries

Signed-off-by: Flavio Ceolin <flavio.ceolin@intel.com>

.gitignore

@@ -8,6 +8,7 @@
 *.swo
 *~
 build*/
+!doc/guides/build
 cscope.*
 .dir
 

doc/guides/build/index.rst

@@ -0,0 +1,155 @@
+.. _build_overview:
+
+Build Overview
+##############
+
+The Zephyr build process can be divided into two main phases: a
+configuration phase (driven by *CMake*) and a build phase (driven by
+*Make* or *Ninja*). We will descibe the build phase using *Make* as
+example.
+
+
+Configuration Phase
+*******************
+
+The configuration phase begins when the user invokes *CMake*,
+specifying a source application directory and a board target.
+
+.. figure:: build-config-phase.svg
+    :align: center
+    :alt: Zephyr's build configuration phase
+    :figclass: align-center
+    :width: 80%
+
+*CMake* begins by processing the *CMakeLists.txt* file in the application
+directory, which refers to the *CMakeLists.txt* file in the Zephyr
+top-level directory, which in turn refers to *CMakeLists.txt* files
+throughout the build tree (directly and indirectly). Its primary
+output is a set of Makefiles  to drive the build process, but *CMake*
+scripts do some build processing of their own:
+
+Device tree
+   Using *cpp*, device-tree specifications (*.dts/.dtsi* files) are
+   collected from the target’s architecture, SoC, board, and
+   application directories and compiled with *dtc*. Then the build
+   tool (scripts/dts) convert this into *.h* files for later
+   consumption.
+
+Device tree fixup
+   Files named *dts_fixup.h* from the target’s architecture, SoC,
+   board, and application directories are concatenated into a single
+   *dts_fixup.h*. Its purpose is to normalize constants output in the
+   previous step so they have the names expected by the source files
+   in the build phase.
+
+Kconfig
+   The build tool reads the *Kconfig* files for the target
+   architecture, the target SoC, the target board, the target
+   application, as well as *Kconfig* files associated with subsystems
+   throughout the source tree. It incorporates the device tree outputs
+   to allow configurations to make use of that data. It ensures the
+   desired configuration is consistent, outputs *autoconf.h* for the
+   build phase.
+
+Build Phase
+***********
+
+The build phase begins when the user invokes *make*. Its ultimate
+output is a complete Zephyr application in a format suitable for
+loading/flashing on the desired target board (*zephyr.elf*,
+*zephyr.hex*, etc.) The build phase can be broken down, conceptually,
+into four stages: the pre-build, first-pass binary, final binary, and
+post-processing.
+
+Pre-build occurs before any source files are compiled, because during
+this phase header files used by the source files are generated.
+
+Pre-build
+=========
+
+Offset generation
+   Access to high-level data structures and members is sometimes
+   required when the definitions of those structures is not
+   immediately accessible (e.g., assembly language). The generation of
+   *offsets.h* (by *gen_offset_header.py*) facilitates this.
+
+System call boilerplate
+   The *gen_syscall_header.py*, *parse_syscalls.py* and
+   *gen_syscall_header.py* scripts work together to bind potential
+   system call functions with their implementations.
+
+.. figure:: build-build-phase-1.svg
+    :align: center
+    :alt: Zephyr's build stage I
+    :figclass: align-center
+    :width: 80%
+
+First-pass binary
+=================
+
+Compilation proper begins with the first-pass binary. Source files (C
+and assembly) are collected from various subsystems (which ones is
+decided during the configuration phase), and compiled into archives
+(with reference to header files in the tree, as well as those
+generated during the configuration phase and the pre-build stage).
+
+If memory protection is enabled, then:
+
+Partition grouping
+   The gen_app_partitions.py script scans all the
+   generated archives and outputs linker scripts to ensure that
+   application partitions are properly grouped and aligned for the
+   target’s memory protection hardware.
+
+Then *cpp* is used to combine linker script fragments from the target’s
+architecture/SoC, the kernel tree, optionally the partition output if
+memory protection is enabled, and any other fragments selected during
+the configuration process, into a *linker.cmd* file. The compiled
+archives are then linked with *ld* as specified in the
+*linker.cmd*.
+
+In some configurations, this is the final binary, and the next stage
+is skipped.
+
+.. figure:: build-build-phase-2.svg
+    :align: center
+    :alt: Zephyr's build stage II
+    :figclass: align-center
+    :width: 80%
+
+Final binary
+============
+
+In some configurations, the binary from the previous stage is
+incomplete, with empty and/or placeholder sections that must be filled
+in by, essentially, reflection. When :ref:`usermode` is enabled:
+
+Kernel object hashing
+   The *gen_kobject_list.py* scans the *ELF DWARF*
+   debug data to find the address of the all kernel objects. This
+   list is passed to *gperf*, which generates a perfect hash function and
+   table of those addresses, then that output is optimized by
+   *process_gperf.py*, using known properties of our special case.
+
+Then, the link from the previous stage is repeated, this time with the
+missing pieces populated.
+
+.. figure:: build-build-phase-3.svg
+    :align: center
+    :alt: Zephyr's build stage III
+    :figclass: align-center
+    :width: 80%
+
+
+Post processing
+===============
+
+Finally, if necessary, the completed kernel is converted from *ELF* to
+the format expected by the loader and/or flash tool required by the
+target. This is accomplished in a straightforward manner with *objdump*.
+
+.. figure:: build-build-phase-4.svg
+    :align: center
+    :alt: Zephyr's build final stage
+    :figclass: align-center
+    :width: 80%

doc/guides/index.rst

@@ -30,3 +30,4 @@ User and Developer Guides
    tracing/index
    west/index
    optimizations/index
+   build/index