use similar filtering logic as with USB drives to provide
the user a more sane list of boot device options. Show user
only valid bootable partitions, not block devices.
There's no point in showing /dev/nvme0 and /dev/nvme0n1 (eg)
when /dev/nvme0n1p[1..n] (eg) exist, as the former are not
valid boot devices.
Signed-off-by: Matt DeVillier <matt.devillier@puri.sm>