[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/x86/core/x86_string.c

@@ -22,6 +22,8 @@
  *
  */
 
+FILE_LICENCE ( GPL2_OR_LATER );
+
 #include <string.h>
 
 /**