Merge UEFI index update

Allow having a larger variable index than the RPC buffer size. This
enables decreasing the memory footprint of the SmmGW SP.

Save and load the variable index in chunks, and use two index variables
to ensure fail-safe operation.

Change-Id: I23671c5735a4a6e5420e69407923d4b562d69b13
Signed-off-by: Gyorgy Szing <gyorgy.szing@arm.com>