cgpt: Add a callback to allow override of GPT entry priority

This can be used by implementations that want to request vboot to
favor a particular kernel entry for booting without affecting the
checks for rollback protection and image verification.

CQ-DEPEND=CL:274716, CL:274932, CL:275171
BUG=None
BRANCH=None
TEST=Compiles successfully. make -j runtests successful.

Change-Id: I6a4600020354f5d4118c17f083c353c2585c4181
Signed-off-by: Furquan Shaikh <furquan@google.com>
Reviewed-on: https://chromium-review.googlesource.com/274558
Reviewed-by: Furquan Shaikh <furquan@chromium.org>
Reviewed-by: Stefan Reinauer <reinauer@chromium.org>
Tested-by: Nicolas Boichat <drinkcat@chromium.org>
Commit-Queue: Nicolas Boichat <drinkcat@chromium.org>
Trybot-Ready: Nicolas Boichat <drinkcat@chromium.org>
5 files changed