[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/arch/x86/prefix/efiprefix.c

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