cmake: kconfig: allow build systems to decide defconfig

Allow build system to define an alternative defconfig file.
This allows reusing the existing kconfig.cmake file in other projects,
such as multi image builds.

Signed-off-by: Torsten Rasmussen <[email protected]>

Author: tejlmand

cmake/modules/kconfig.cmake

@@ -59,7 +59,7 @@ else()
   set(KCONFIG_ROOT ${ZEPHYR_BASE}/Kconfig)
 endif()
 
-set(BOARD_DEFCONFIG ${BOARD_DIR}/${BOARD}_defconfig)
+set_ifndef(BOARD_DEFCONFIG ${BOARD_DIR}/${BOARD}_defconfig)
 set(DOTCONFIG                  ${PROJECT_BINARY_DIR}/.config)
 set(PARSED_KCONFIG_SOURCES_TXT ${PROJECT_BINARY_DIR}/kconfig/sources.txt)