[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/eepro.c

@@ -31,6 +31,8 @@ has 34 pins, the top row of 2 are not used.
  * your option) any later version.
  */
 
+FILE_LICENCE ( GPL2_OR_LATER );
+
 #include "etherboot.h"
 #include <errno.h>
 #include "nic.h"