[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/r8169.c

@@ -23,6 +23,8 @@
  * Copyright (c) a lot of people too. Please respect their work.
  */
 
+FILE_LICENCE ( GPL2_OR_LATER );
+
 #include <stdint.h>
 #include <stdio.h>
 #include <stdlib.h>