[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/strings.h

@@ -1,6 +1,8 @@
 #ifndef _STRINGS_H
 #define _STRINGS_H
 
+FILE_LICENCE ( GPL2_OR_LATER );
+
 #include <limits.h>
 #include <string.h>