[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/i386/core/relocate.c

@@ -9,6 +9,8 @@
  *
  */
 
+FILE_LICENCE ( GPL2_OR_LATER );
+
 /*
  * The linker passes in the symbol _max_align, which is the alignment
  * that we must preserve, in bytes.