Silicon/FitGen: Add FitGen feature to support uCode Capsule Update

Add slot mode handling with a header array.
BZ: https://bugzilla.tianocore.org/show_bug.cgi?id=2321



Signed-off-by: Chen A Chen <chen.a.chen@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>