[pxe] Add startpxe and stoppxe commands

[pxe] Add startpxe and stoppxe commands

These commands can be used to activate or deactivate the PXE API (on a
specifiable network interface).

This is currently of limited use, since most image formats will call
shutdown() before booting the image, meaning that the underlying net
device gets shut down during remove_devices() anyway.