build: allow platform-defined flags

Similarly to the earlier patch enabling BL-specific additions to include
directories, preprocessor definitions and toolchain flags, this change
allows platforms to add options common to all images.

This is required because some platforms inject dependencies via the
`<platform_def.h>` header, and we don't currently have a clean way to
model that in build system code.

Change-Id: Ib0b38f9236cba6f56745cb3c756dfc81547da8bd
Signed-off-by: Chris Kay <chris.kay@arm.com>
1 file changed