[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/include/stdarg.h

@@ -1,6 +1,8 @@
 #ifndef _STDARG_H
 #define _STDARG_H
 
+FILE_LICENCE ( GPL2_OR_LATER );
+
 typedef __builtin_va_list va_list;
 #define va_start( ap, last ) __builtin_va_start ( ap, last )
 #define va_arg( ap, type ) __builtin_va_arg ( ap, type )