build(bl2): deduplicate sources

Deduplicating sources prevents the build system from complaining about
multiply-compiled files, which can happen if multiple makefiles depend
on a component. This already occurs for BL31.

Signed-off-by: Chris Kay <chris.kay@arm.com>
Change-Id: Ic9e67932550f07cb9e4d199f68bc46c33a611748
1 file changed