Add a new sample that supports hybrid boot, i.e. legacy boot and
UEFI, like f30-hybrid-qcow2, but with a separate boot partition.
The latter means that 'fix-bls' as well as the 'grub2' stage need
to be adapted for it: 'fix-bls' needs have the prefix set to "/"
and grub2 needs to have a 'boot_fs_uuid' option pointing to the
uuid of the filesystem for the boot partition.