[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/core/serial.c

@@ -11,6 +11,8 @@
  * parity, 1 stop bit (8N1).  This can be changed in init_serial().
  */
 
+FILE_LICENCE ( GPL2_OR_LATER );
+
 #include "stddef.h"
 #include <gpxe/init.h>
 #include <gpxe/io.h>