cmd_boot: Add a poweroff command

Add a 'poweroff' command to boot commands, this only gets enabled if the
board Kconfig does a "select CMD_POWEROFF".

Signed-off-by: Michael van Slingerland <michael@deviousops.nl>
[hdegoede@redhat.com: Make the cmd conditional on a CMD_POWEROFF Kconfig]
Signed-off-by: Hans de Goede <hdegoede@redhat.com>