mmu: add CONFIG_SRAM_REGION_PERMISSIONS

If CONFIG_MMU is active, choose whether to separate text,
rodata, and ram into their own page-aligned regions so that
they have have different MMU permissions applied.

If disabled, all RAM pages will have RWX permission to
supervisor mode, but some memory may be saved due to lack
of page alignment padding between these regions.

This used to always happen. This patch adds the Kconfig,
linker script changes to come in a subsequent patch.

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>

arch/Kconfig

@@ -208,6 +208,7 @@ config USERSPACE
 	bool "User mode threads"
 	depends on ARCH_HAS_USERSPACE
 	depends on RUNTIME_ERROR_CHECKS
+	select SRAM_REGION_PERMISSIONS if MMU
 	help
 	  When enabled, threads may be created or dropped down to user mode,
 	  which has significantly restricted permissions and must interact
@@ -507,6 +508,17 @@ config MMU_PAGE_SIZE
 	  Size of memory pages. Varies per MMU but 4K is common. For MMUs that
 	  support multiple page sizes, put the smallest one here.
 
+config SRAM_REGION_PERMISSIONS
+	bool "Assign appropriate permissions to kernel areas in SRAM"
+	depends on MMU
+	default y
+	help
+	  If enabled, the program text, rodata, and data parts of the kernel in
+	  the permanent mappings created at build time will have appropriate
+	  permissions set. Uses extra memory due to page-alignment constraints.
+	  If not enabled, all SRAM mappings will allow supervisor mode to
+	  read, write, and execute. User mode support requires this.
+
 config MEMORY_PROTECTION
 	bool
 	help