lib: Add EFI support to {check,set}_bootcounter

aum-rollback-blacklist test fails on UEFI AMD64 targets with log:

+ check_bootcounter   00 01
+ + read -r m ver cnt uflag
od -t x1 -An /boot/uboot.cnt
od: '/boot/uboot.cnt': No such file or directory (os error 2)

For EFI, the bootcounter is stored in the loader entry filename, i.e. /boot/efi/loader/entries/ostree-0-2[+bootleft[-bootcount]].conf.

https://phabricator.apertis.org/T8409

Signed-off-by: Frédéric Danis frederic.danis@collabora.com