[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/assert.h

@@ -10,6 +10,8 @@
  *
  */
 
+FILE_LICENCE ( GPL2_OR_LATER );
+
 #ifdef NDEBUG
 #define ASSERTING 0
 #else