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>
4 files changed