[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/drivers/net/tlan.c

@@ -38,6 +38,8 @@
 *    Indent Options: indent -kr -i8
 ***************************************************************************/
 
+FILE_LICENCE ( GPL2_OR_LATER );
+
 #include "etherboot.h"
 #include "nic.h"
 #include <gpxe/pci.h>