[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/i386/include/pxe_api.h

@@ -23,6 +23,8 @@
  *
  */
 
+FILE_LICENCE ( GPL2_OR_LATER );
+
 #include "pxe_types.h"
 
 /** @addtogroup pxe Preboot eXecution Environment (PXE) API