With this commit a dedicated APPLICATION_CONFIG_DIR is added to the
Zephyr build system.
Currently, the APPLICATION_SOURCE_DIR is identical also the base
location of configuration files.
This is very practical for simple samples, but also has it limitations
for more complex setups.
Introducing a dedicated APPLICATION_CONFIG_DIR allows users more
customization options in Zephyr build system.
Especially in terms of custom build configuration files unknown to
Zephyr itself.
For example, instead of all configuration files being located directly
in the application source folder, a project might prefer to organize
their configuration files on a per board basis, for example:
<app>/boards/custom_board_A/prj.conf
<app>/boards/custom_board_A/app.overlay
<app>/boards/custom_board_A/custom_file.mine
<app>/boards/custom_board_B/prj.conf
<app>/boards/custom_board_B/app.overlay
<app>/boards/custom_board_B/custom_file.mine
...
instead of n-files located in the root of the sample.
If the user / sample specifies APPLICATION_CONFIG_DIR, then this folder
will always be used instead of the default configuration folder.
As part of this extension the behaviour of
`-DCONF_FILE=<relative-path>/prj_<build>.conf` and additional Kconfig
fragments automatic sourcing has been aligned with the default behavior
of `prj.conf`.
Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>
