[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/arch/i386/Makefile

 SRCDIRS		+= arch/i386/interface/pcbios
 SRCDIRS		+= arch/i386/interface/pxe
 SRCDIRS 	+= arch/i386/interface/syslinux
+SRCDIRS		+= arch/i386/hci/commands
 
 # The various xxx_loader.c files are #included into core/loader.c and
 # should not be compiled directly.

src/arch/i386/hci/commands/pxe_cmd.c

+#include <gpxe/netdevice.h>
+#include <gpxe/command.h>
+#include <hci/ifmgmt_cmd.h>
+#include <pxe_call.h>
+
+FILE_LICENCE ( GPL2_OR_LATER );
+
+static int startpxe_payload ( struct net_device *netdev ) {
+	if ( netdev->state & NETDEV_OPEN )
+		pxe_activate ( netdev );
+	return 0;
+}
+
+static int startpxe_exec ( int argc, char **argv ) {
+	return ifcommon_exec ( argc, argv, startpxe_payload,
+			       "Activate PXE on" );
+}
+
+static int stoppxe_exec ( int argc __unused, char **argv __unused ) {
+	pxe_deactivate();
+	return 0;
+}
+
+struct command pxe_commands[] __command = {
+	{
+		.name = "startpxe",
+		.exec = startpxe_exec,
+	},
+	{
+		.name = "stoppxe",
+		.exec = stoppxe_exec,
+	},
+};

src/config/general.h

 #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

src/core/config.c

 #ifdef DIGEST_CMD
 REQUIRE_OBJECT ( digest_cmd );
 #endif
+#ifdef PXE_CMD
+REQUIRE_OBJECT ( pxe_cmd );
+#endif
 
 /*
  * Drag in miscellaneous objects