fs: partitions: efi: Add force_gpt_sector parameter

fs: partitions: efi: Add force_gpt_sector parameter

force_gpt_sector=<sector> causes the GPT partition search to
look at the specified sector for a valid GPT header if the
GPT is not found at the beginning or the end of the block
device.

Change-Id: I9b5f85ce24719c0538d42ec5a94344c7f6556b2b
Signed-off-by: Colin Cross <ccross@android.com>
Rebase-Id: Rbcbb8f2f0882f3750e748b5cc83038ca0f5940db
Change-Id: I37c031d8d8e3268a900a7dee2371846e2308db22
Reviewed-on: http://git-master/r/1260443
GVS: Gerrit_Virtual_Submit
Reviewed-by: Bharat Nihalani <bnihalani@nvidia.com>
Tested-by: Leo Chiu <lchiu@nvidia.com>