[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.

src/config/general.h

@@ -105,6 +105,7 @@ FILE_LICENCE ( GPL2_OR_LATER );
 #define LOGIN_CMD		/* Login command */
 #undef	TIME_CMD		/* Time commands */
 #undef	DIGEST_CMD		/* Image crypto digest commands */
+#define	PXE_CMD			/* PXE commands */
 
 /*
  * Obscure configuration options