commands/search: Add new --efidisk-only option for EFI systems

commands/search: Add new --efidisk-only option for EFI systems

When using "search" on EFI systems, we sometimes want to exclude devices
that are not EFI disks, e.g. md, lvm. This is typically used when
wanting to chainload when having a software raid (md) for EFI partition:
with no option, "search --file /EFI/redhat/shimx64.efi" sets root envvar
to "md/boot_efi" which cannot be used for chainloading since there is no
effective EFI device behind.

Signed-off-by: Renaud Métrich <rmetrich@redhat.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>