[legal] Add a selection of FILE_LICENCE declarations

Add FILE_LICENCE declarations to almost all files that make up the
various standard builds of gPXE.

src/hci/commands/autoboot_cmd.c

@@ -2,6 +2,8 @@
 #include <gpxe/command.h>
 #include <usr/autoboot.h>
 
+FILE_LICENCE ( GPL2_OR_LATER );
+
 static int autoboot_exec ( int argc, char **argv ) {
 
 	if ( argc != 1 ) {

src/hci/commands/config_cmd.c

@@ -4,6 +4,8 @@
 #include <gpxe/settings.h>
 #include <gpxe/settings_ui.h>
 
+FILE_LICENCE ( GPL2_OR_LATER );
+
 static int config_exec ( int argc, char **argv ) {
 	char *settings_name;
 	struct settings *settings;

src/hci/commands/dhcp_cmd.c

@@ -16,6 +16,8 @@
  * Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
  */
 
+FILE_LICENCE ( GPL2_OR_LATER );
+
 #include <stdio.h>
 #include <stdint.h>
 #include <stdlib.h>

src/hci/commands/ifmgmt_cmd.c

@@ -16,6 +16,8 @@
  * Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
  */
 
+FILE_LICENCE ( GPL2_OR_LATER );
+
 #include <stdio.h>
 #include <getopt.h>
 #include <gpxe/netdevice.h>

src/hci/commands/image_cmd.c

@@ -16,6 +16,8 @@
  * Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
  */
 
+FILE_LICENCE ( GPL2_OR_LATER );
+
 #include <stdint.h>
 #include <stdlib.h>
 #include <stdio.h>

src/hci/commands/login_cmd.c

@@ -3,6 +3,8 @@
 #include <gpxe/command.h>
 #include <gpxe/login_ui.h>
 
+FILE_LICENCE ( GPL2_OR_LATER );
+
 static int login_exec ( int argc, char **argv ) {
 	int rc;
 

src/hci/commands/nvo_cmd.c

@@ -7,6 +7,8 @@
 #include <gpxe/settings.h>
 #include <gpxe/command.h>
 
+FILE_LICENCE ( GPL2_OR_LATER );
+
 static int show_exec ( int argc, char **argv ) {
 	char buf[256];
 	int rc;

src/hci/commands/route_cmd.c

@@ -16,6 +16,8 @@
  * Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
  */
 
+FILE_LICENCE ( GPL2_OR_LATER );
+
 #include <stdio.h>
 #include <getopt.h>
 #include <gpxe/command.h>

src/hci/commands/sanboot_cmd.c

@@ -4,6 +4,8 @@
 #include <gpxe/command.h>
 #include <usr/autoboot.h>
 
+FILE_LICENCE ( GPL2_OR_LATER );
+
 /**
  * "sanboot" command syntax message
  *