discover/grub: Use different paths to search for the BLS directory

discover/grub: Use different paths to search for the BLS directory

Currenlty the BLS fragments are only searched in the /loader/entries
directory, but this assumes that there is a boot partition mounted
in /boot. This may not always be the case, /boot may not be a mount
point and just a directory inside the root partition.

To cover this case, Petitboot tries to find a GRUB 2 config file in
different paths. So let's do the same for the BLS files directory.

Also change some of the unit tests to use /boot/loader/entries as a
BLS directory instead of /loader/entries.

Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Signed-off-by: Samuel Mendoza-Jonas <sam@mendozajonas.com>