Fix the path to bootable binaries in efi entry

`stat` doesn't work well due `coreutils-gplv2`
created by Debos returning "?" instead of mount point.
Fall back to the provided directory, since we have separate
partition for "/boot/efi".
This change allow to use correct paths for kernel and initrd
for entry generation.

Signed-off-by: Denis Pynkin <denis.pynkin@collabora.com>