With this we can do 'boot <name>' where name is one of:

 - a filename under /env/boot/
 - a full path to a boot script
 - a device name
 - a partition name under /dev/
 - a full path to a directory which
   - contains boot scripts, or
   - contains a loader/entries/ directory containing bootspec entries

Multiple names can be given, they are tried in order. So any mixture
between bootspec entries and bootscripts can be given. bootspec entries
can now also be given as a path to a directory containing bootspec entries.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>