app: Add example of VERSION file tracking git index file

Adds an example of how to add the git index file (if this is inside
of a git repo) to the dependencies of the application version file,
which allows the git commit of the application version to be
updated as new commits are added

Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>

Author: nordicjm

app/CMakeLists.txt

@@ -10,3 +10,27 @@ find_package(Zephyr REQUIRED HINTS $ENV{ZEPHYR_BASE})
 project(app LANGUAGES C)
 
 target_sources(app PRIVATE src/main.c)
+
+# The code below locates the git index file for this repository and adds it as a dependency for
+# the application VERSION file so that if the repo has a new commit added, even if no files in
+# the build have changed, the application version file will be regenerated with the new git commit
+find_package(Git QUIET)
+if(GIT_FOUND)
+  execute_process(
+    COMMAND ${GIT_EXECUTABLE} rev-parse --absolute-git-dir
+    WORKING_DIRECTORY .
+    OUTPUT_VARIABLE application_git_dir
+    OUTPUT_STRIP_TRAILING_WHITESPACE
+    ERROR_STRIP_TRAILING_WHITESPACE
+    ERROR_VARIABLE stderr
+    RESULT_VARIABLE return_code)
+  # If there is an error e.g. it is not a git repo, we just silently ignore it and continue
+  # without a dependency, this will be the case with freestanding applications.
+  if(NOT return_code)
+    if(NOT "${stderr}" STREQUAL "")
+      message(WARNING "APPLICATION_BUILD_VERSION: git rev-parse warned: ${stderr}")
+    endif()
+
+    set_property(TARGET app_version_h PROPERTY APP_VERSION_DEPENDS ${application_git_dir}/index)
+  endif()
+endif()