[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/core/string.c

@@ -10,6 +10,8 @@
  * published by the Free Software Foundation.
  */
 
+FILE_LICENCE ( GPL2_ONLY );
+
 /*
  * stupid library routines.. The optimized versions should generally be found
  * as inline code in <asm-xx/string.h>