Forward-port gptpriority command to GRUB 2.00

Forward-ported from git commit 9866f9750a38a175849ca04439922ac1ffe8a0e7
with significant edits: Ported from the previous Ruby-based build system
to the new autogen-based Makefile.am generator, and updated for GRUB API
changes.  In particular, the previous version of gptpriority counted on
GRUB's part_gpt to provide access to the original partition table data
in a field of "struct grub_partition" that no longer exists; this
updated version modifies part_gpt to directly export the relevant
attribute field as a new field in grub_partition.

Since this adds a new source file and a new module, and the GRUB ebuild
expects source in "make dist" form (with configure and Makefile.in
already generated), this commit also updates the corresponding generated
files via autogen.sh.

BUG=chromium:265918
TEST=With the complete patch series, built a complete x86-generic image,
     and booted it via UEFI on both 32-bit and 64-bit UEFI platforms.

CQ-DEPEND=Ie865f3d5ab0ecd8e9138bfd5b576a8147a205757

Change-Id: I238f4ad26af3b1d924d5d5ac5fbcd567415cfb3f
Reviewed-on: https://gerrit.chromium.org/gerrit/63986
Tested-by: Josh Triplett <josh@joshtriplett.org>
Reviewed-by: Liam McLoughlin <lmcloughlin@chromium.org>
Commit-Queue: Josh Triplett <josh@joshtriplett.org>
6 files changed