tbbr/dualroot: rename SP package certificate file

Currently only single signing domain is supported for SP packages but
there is plan to support dual signing domains if CoT is dualroot.

SP_CONTENT_CERT_ID is the certificate file which is currently generated
and signed with trusted world key which in-turn is derived from Silicon
provider RoT key.
To allow dual signing domain for SP packages, other certificate file
will be derived from Platform owned RoT key.

This patch renames "SP_CONTENT_CERT_ID" to "SIP_SP_CONTENT_CERT_ID" and
does other related changes.

Signed-off-by: Manish Pandey <manish.pandey2@arm.com>
Change-Id: I0bc445a3ab257e2dac03faa64f46e36a9fed5e93