Some grub configs/bls entries contain the full paths to the
kernel/initrd files, which the parsers currently fail to handle,
causing a failed boot without any useful error being presented to the user.
To fix this, strip the bootdir prefix from the menu entries when parsing,
should it exist.
Test: build/boot Librem 13v2 w/F32 and bls entries containing absolute paths.
Signed-off-by: Matt DeVillier <matt.devillier@puri.sm>
