[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/dmfe.c

@@ -36,6 +36,8 @@
 *
 ***************************************************************************/
 
+FILE_LICENCE ( GPL2_OR_LATER );
+
 /* to get some global routines like printf */
 #include "etherboot.h"
 /* to get the interface to the body of the program */