[cmdline] Add "poweroff" command

Modified-by: Michael Brown <mcb30@ipxe.org>
Signed-off-by: Marin Hannache <git@mareo.fr>
Signed-off-by: Michael Brown <mcb30@ipxe.org>
[build] Include ipxe.pxe in default build

Signed-off-by: Robin Smidsrød <robin@smidsrod.no>
Signed-off-by: Michael Brown <mcb30@ipxe.org>