scripts: move dir_is_writeable.py to scripts/build

Move scripts needed by the build system and not designed to be run
individually or standalone into the build subfolder.

Signed-off-by: Anas Nashif <anas.nashif@intel.com>

cmake/modules/user_cache.cmake

@@ -62,7 +62,7 @@ function(find_appropriate_cache_directory dir)
       set(test_user_dir ${env_dir}/${env_suffix_${env_var}})
 
       execute_process(COMMAND ${PYTHON_EXECUTABLE}
-        ${ZEPHYR_BASE}/scripts/dir_is_writeable.py ${test_user_dir}
+        ${ZEPHYR_BASE}/scripts/build/dir_is_writeable.py ${test_user_dir}
         RESULT_VARIABLE writable_result
       )
       if("${writable_result}" STREQUAL "0")