partition_parser: Update logic to access backup GPT header

Change to access backup GPT header from the last LBA when primary
GPT header is corrupted instead of corrupted primary GPT header.
Populate the start of the partition entry array using the PARTITION_
ENTRY_OFFSET from the valid GPT header.

CRs-Fixed: 342475

Change-Id: I4a5501103894c4ad2327b72e7df0dea87dffcd27
1 file changed