sysbuild: Add flag to customize variant app config

Add a flag to allow for customizing the variant application Kconfig
values.

Ref: NCSDK-30812

Signed-off-by: Tomasz Chyrowicz <tomasz.chyrowicz@nordicsemi.no>

cmake/modules/kconfig.cmake

@@ -7,24 +7,31 @@
 include_guard(GLOBAL)
 
 if(CONFIG_NCS_IS_VARIANT_IMAGE)
-  # A variant build should reuse same .config and thus autoconf.h, therefore
-  # copy files from original and bypass Kconfig invocation.
   set(AUTOCONF_H ${PROJECT_BINARY_DIR}/include/generated/zephyr/autoconf.h)
   set(DOTCONFIG  ${PROJECT_BINARY_DIR}/.config)
 
-  set(preload_autoconf_h ${PRELOAD_BINARY_DIR}/zephyr/include/generated/zephyr/autoconf.h)
-  set(preload_dotconfig  ${PRELOAD_BINARY_DIR}/zephyr/.config)
-
-  file(COPY ${preload_dotconfig}  DESTINATION ${PROJECT_BINARY_DIR})
-  file(COPY ${preload_autoconf_h} DESTINATION ${PROJECT_BINARY_DIR}/include/generated/zephyr)
-  file(APPEND ${AUTOCONF_H} "#define CONFIG_NCS_IS_VARIANT_IMAGE 1")
-
   set_property(DIRECTORY APPEND PROPERTY CMAKE_CONFIGURE_DEPENDS ${AUTOCONF_H})
   set_property(DIRECTORY APPEND PROPERTY CMAKE_CONFIGURE_DEPENDS ${DOTCONFIG})
-  set_property(DIRECTORY APPEND PROPERTY CMAKE_CONFIGURE_DEPENDS ${preload_autoconf_h})
-  set_property(DIRECTORY APPEND PROPERTY CMAKE_CONFIGURE_DEPENDS ${preload_dotconfig})
 
-  import_kconfig("CONFIG" ${DOTCONFIG})
+  if(CONFIG_NCS_VARIANT_MERGE_KCONFIG)
+    # A variant build should reuse same .config and thus autoconf.h, therefore
+    # copy files from original and bypass Kconfig invocation.
+    set(preload_autoconf_h ${PRELOAD_BINARY_DIR}/zephyr/include/generated/zephyr/autoconf.h)
+    set(preload_dotconfig  ${PRELOAD_BINARY_DIR}/zephyr/.config)
+
+    file(COPY ${preload_dotconfig}  DESTINATION ${PROJECT_BINARY_DIR})
+    file(COPY ${preload_autoconf_h} DESTINATION ${PROJECT_BINARY_DIR}/include/generated/zephyr)
+
+    set_property(DIRECTORY APPEND PROPERTY CMAKE_CONFIGURE_DEPENDS ${preload_autoconf_h})
+    set_property(DIRECTORY APPEND PROPERTY CMAKE_CONFIGURE_DEPENDS ${preload_dotconfig})
+
+    import_kconfig("CONFIG" ${DOTCONFIG})
+  else()
+    # First generate autoconf.h as well as .config files.
+    include(${ZEPHYR_BASE}/cmake/modules/kconfig.cmake)
+  endif()
+
+  file(APPEND ${AUTOCONF_H} "#define CONFIG_NCS_IS_VARIANT_IMAGE 1")
 else()
   include(${ZEPHYR_BASE}/cmake/modules/kconfig.cmake)
 endif()

cmake/sysbuild/modules/ncs_sysbuild_extensions.cmake

@@ -16,8 +16,10 @@ include_guard(GLOBAL)
 # APPLICATION <name>: Name of the application which is used as the source for
 #                     the variant build.
 # VARIANT <name>:     Name of the variant build.
+# SPLIT_KCONFIG:      Flag indicating that the variant image should
+#                     not reuse the same .config files.
 function(ExternalNcsVariantProject_Add)
-  cmake_parse_arguments(VBUILD "" "APPLICATION;VARIANT" "" ${ARGN})
+  cmake_parse_arguments(VBUILD "SPLIT_KCONFIG" "APPLICATION;VARIANT" "" ${ARGN})
 
   ExternalProject_Get_Property(${VBUILD_APPLICATION} SOURCE_DIR BINARY_DIR)
   set(${VBUILD_APPLICATION}_BINARY_DIR ${BINARY_DIR})
@@ -41,9 +43,15 @@ function(ExternalNcsVariantProject_Add)
   set_property(TARGET ${VBUILD_VARIANT} PROPERTY NCS_VARIANT_APPLICATION ${VBUILD_APPLICATION})
   set_property(TARGET ${VBUILD_VARIANT} APPEND PROPERTY _EP_CMAKE_ARGS
     -DCONFIG_NCS_IS_VARIANT_IMAGE=y
-    -DPRELOAD_BINARY_DIR=${${VBUILD_APPLICATION}_BINARY_DIR}
   )
 
+  if(NOT VBUILD_SPLIT_KCONFIG)
+    set_property(TARGET ${VBUILD_VARIANT} APPEND PROPERTY _EP_CMAKE_ARGS
+      -DPRELOAD_BINARY_DIR=${${VBUILD_APPLICATION}_BINARY_DIR}
+      -DCONFIG_NCS_VARIANT_MERGE_KCONFIG=y
+    )
+  endif()
+
   # Configure variant image after application so that the configuration is present
   sysbuild_add_dependencies(CONFIGURE ${VBUILD_VARIANT} ${VBUILD_APPLICATION})
 endfunction()