cgpt: mark EFI partition as bootable in the GPT

BUG=chromium:644845
TEST=built an amd64-generic image and it booted via USB on a new generic x86 laptop
CQ-DEPEND=CL:382411

Change-Id: I3dce104a8f853a8fbca1c14ba5d3dd111d47df0a
Reviewed-on: https://chromium-review.googlesource.com/382472
Commit-Ready: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Randall Spangler <rspangler@chromium.org>
1 file changed