BaseTools/GenFv: Support SecCore and PeiCore in different FV

BaseTools/GenFv: Support SecCore and PeiCore in different FV

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=1423

There is usage model that SecCore and PeiCore are in different FVs.
Update BaseTools to support this usage model.

Test: Verified on internal platform with the case SecCore and
PeiCore in different FVs and built/booted successfully.

Cc: Bob Feng <bob.c.feng@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Cc: Yonghong Zhu <yonghong.zhu@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Chasel Chiu <chasel.chiu@intel.com>
Reviewed-by: Bob Feng <bob.c.feng@intel.com>