cmake: Add function for checking if a directory is write-able

Added a CMake extention for determining if a directory is write-able
by the build system. To determine this, we attempt to 'touch' a file
in the directory and check the return code of this command.

Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>

cmake/extensions.cmake

@@ -18,6 +18,7 @@ include(CheckCXXCompilerFlag)
 # 3.2. *_ifndef
 # 3.3. *_option compiler compatibility checks
 # 3.4. Debugging CMake
+# 3.5. File system management
 
 ########################################################
 # 1. Zephyr-aware extensions
@@ -1004,3 +1005,21 @@ macro(assert_with_usage test comment)
   endif()
 endmacro()
 
+# 3.5. File system management
+function(check_if_directory_is_writeable dir ok)
+  execute_process(
+    COMMAND
+    ${PYTHON_EXECUTABLE}
+    ${ZEPHYR_BASE}/scripts/dir_is_writeable.py
+    ${dir}
+    RESULT_VARIABLE ret
+    )
+
+  if("${ret}" STREQUAL "0")
+    # The directory is write-able
+    set(${ok} 1 PARENT_SCOPE)
+  else()
+    set(${ok} 0 PARENT_SCOPE)
+  endif()
+endfunction()
+

scripts/dir_is_writeable.py

@@ -0,0 +1,10 @@
+import os
+import sys
+
+def main():
+    is_writeable = os.access(sys.argv[1], os.W_OK)
+    return_code = int(not is_writeable)
+    sys.exit(return_code)
+
+if __name__ == "__main__":
+    main()