[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/stdint.h

@@ -1,6 +1,8 @@
 #ifndef _STDINT_H
 #define _STDINT_H
 
+FILE_LICENCE ( GPL2_OR_LATER );
+
 /*
  * This is a standard predefined macro on all gcc's I've seen. It's
  * important that we define size_t in the same way as the compiler,