Fixes: #40389
This commit implements zephyr_get(<variable>).
The purpose of zephyr_get(<variable>) is to ensure a uniform way to
handle variables which can be defined as:
- CMake cache variable, for example `-D<var>=<value>`.
- Environment variable
- Locally in CMakeLists.txt file before `find_package(Zephyr)`
It furthermore ensures that if an environment setting hides a local
setting then a warning is printed.
using
zephyr_get(<var>)
removes the need for constructs like:
set_ifndef(<var> "$ENV{<var>}")
set(<var> ${<var>} CACHE PATH "")
if(NOT DEFINED <var>)
set(<var> $ENV{<var>})
endif()
if (NOT DEFINED ENV{<var>})
message(FATAL_ERROR <error>)
endif()
and also ensures identical preference order for variables used in Zephyr
1. cache
2. environment
3. local scope var
It also ensures the variables supported through environment can also be
set using `-D<var>=<value>`
Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>
90e236581e