This fixes a minor issue in sysbuild, where `-DOVERLAY_CONFIG=...` would
be applied not only to the main application, but also sysbuild itself.
This was incorrect, because sysbuild imports a different Kconfig tree
than normal Zephyr builds, so the same Kconfig fragment file would not
necessarily be valid for both.
This adds a new variable SB_OVERLAY_CONFIG to resolve this ambiguity.
It functions along the lines of SB_CONF_FILE, with both being sysbuild-
specific versions of existing variables.
To ensure that OVERLAY_CONFIG is still passed on to the main application
verbatim, its value is now loaded in `configuration_files.cmake`, rather
than `kconfig.cmake`. This is because the former file is not imported by
sysbuild, and it is where the related variables, such as CONF_FILE and
DTC_OVERLAY_FILE, are loaded as well.
Signed-off-by: Grzegorz Swiderski <grzegorz.swiderski@nordicsemi.no>
af23628e43