RSS: Add separate RSS build to support trusty

* Trusty cannot work with GPT due to flash memory constraints.
* The default RSS build is based on FWU flag and if it is enabled
  RSS builds with GPT. This can break trusty based boot.
* This change is to provide another RSS build to support Trusty while
  keeping the default RSS build to work based on FWU flag.

Signed-off-by: Tintu Thomas <tintu.thomas@arm.com>
Change-Id: I40ed363e17d204fce31c5201c0df4db5b3104ad8