cgptlib: Add functions to cgptlib API

Following changes are done to the cgptlib API in order to allow backend
component of fastboot to perform erase and write operations on
partitions and manipulate GPT entries:
GptFindNthEntry - Returns the nth entry in GPT that matches provided
GUID.
GptGetEntrySizeLba - Returns size of a partition in lba.
GptGetEntrySizeBytes - Returns size of a partition in bytes.
GptUpdateKernelWithEntry - Given kernel entry is updated using the
provided update_type. GptUpdateKernelEntry calls this function with
entry for current_kernel.

Add flags GPT_UPDATE_ENTRY_RESET and GPT_UPDATE_ENTRY_INVALID for
calls to GptUpdateKernelEntry. These operations are used by write
image and erase partition respectively.

BUG=chrome-os-partner:35861
BRANCH=None
TEST=Compiles successfully and all the newly added functions work as expected.

Change-Id: I82c87e4c97de2d207e80209dbd4922b4bcd5880a
Signed-off-by: Furquan Shaikh <furquan@google.com>
Reviewed-on: https://chromium-review.googlesource.com/240268
Trybot-Ready: Furquan Shaikh <furquan@chromium.org>
Tested-by: Furquan Shaikh <furquan@chromium.org>
Reviewed-by: Randall Spangler <rspangler@chromium.org>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Commit-Queue: Furquan Shaikh <furquan@chromium.org>
4 files changed