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/cmake/install.cmake b/cmake/install.cmake
index 3cba192..69db5a3 100644
--- a/cmake/install.cmake
+++ b/cmake/install.cmake
@@ -203,7 +203,4 @@
 include(${CMAKE_SOURCE_DIR}/lib/ext/tf-m-tests/install.cmake)
 
 ##################### Platform-specific installation ###########################
-
-if (EXISTS ${TARGET_PLATFORM_PATH}/install.cmake)
-    include(${TARGET_PLATFORM_PATH}/install.cmake)
-endif()
+include(${TARGET_PLATFORM_PATH}/install.cmake OPTIONAL)