cosboot: Add functionality for determining boot path

For now the boot path can either be grub or UKI. Currently we just use
if the bootx64.efi is the shim. More analysis is possible, but is not
necessary right now. Non-EFI boot paths are not considered.

BUG=b/465210631
TEST=unit tests

Change-Id: Ia9973459958c1d99223607ccf6b5e82b2512b7f1
Reviewed-on: https://cos-review.googlesource.com/c/cos/tools/+/130161
Tested-by: Robert Kolchmeyer <rkolchmeyer@google.com>
Reviewed-by: Kevin Berry <kpberry@google.com>
Cloud-Build: GCB Service account <228075978874@cloudbuild.gserviceaccount.com>
Reviewed-by: He Gao <hegao@google.com>