Build: Use OPTIONAL in include() instead of existence check

Use OPTIONAL in include() instead of file existence checking codes.
This can simply the codes a bit.
If OPTIONAL is present, then no error is raised if the file does not
exist.

Change-Id: Ia22e3154707312d94754e545725a3ba6095c9a13
Signed-off-by: Kevin Peng <kevin.peng@arm.com>
diff --git a/config/check_config.cmake b/config/check_config.cmake
index 482236f..025dbad 100644
--- a/config/check_config.cmake
+++ b/config/check_config.cmake
@@ -120,6 +120,4 @@
 
 ###################### Platform-specific checks ################################
 
-if (EXISTS ${TARGET_PLATFORM_PATH}/check_config.cmake)
-    include(${TARGET_PLATFORM_PATH}/check_config.cmake)
-endif()
+include(${TARGET_PLATFORM_PATH}/check_config.cmake OPTIONAL)