[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/prism2_pci.c

@@ -14,6 +14,8 @@ $Id$
  * your option) any later version.
  */
 
+FILE_LICENCE ( GPL2_OR_LATER );
+
 #include <gpxe/pci.h>
 #include <nic.h>