sysbuild: cmake: modules: kconfig: Fix changed CMake API

Fixes a bug which was caused by the CMake function API having
changed in commit 4e29a35

(cherry picked from commit aed59d4)

Original-Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>
GitOrigin-RevId: aed59d4
Cr-Build-Id: 8727404601633048497
Cr-Build-Url: https://cr-buildbucket.appspot.com/build/8727404601633048497
Copybot-Job-Name: zephyr-main-copybot-downstream
Change-Id: If239c15b0be6562fce2935deca0ea9768ee3fa0f
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/zephyr/+/6126625
Bot-Commit: ChromeOS Prod (Robot) <chromeos-ci-prod@chromeos-bot.iam.gserviceaccount.com>
Commit-Queue: ChromeOS Prod (Robot) <chromeos-ci-prod@chromeos-bot.iam.gserviceaccount.com>
Tested-by: ChromeOS Prod (Robot) <chromeos-ci-prod@chromeos-bot.iam.gserviceaccount.com>

share/sysbuild/cmake/modules/sysbuild_kconfig.cmake

@@ -107,8 +107,9 @@ if(EXISTS ${APP_DIR}/Kconfig.sysbuild)
 endif()
 
 # Apply any EXTRA_CONF_FILE variables from snippets
-if(TARGET snippets_scope)
-  get_property(snippets_EXTRA_CONF_FILE TARGET snippets_scope PROPERTY SB_EXTRA_CONF_FILE)
+zephyr_scope_exists(scope_defined snippets)
+if(scope_defined)
+  zephyr_get_scoped(snippets_EXTRA_CONF_FILE snippets SB_EXTRA_CONF_FILE)
   list(APPEND EXTRA_CONF_FILE ${snippets_EXTRA_CONF_FILE})
 endif()