[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/endian.h

@@ -1,6 +1,8 @@
 #ifndef ETHERBOOT_ENDIAN_H
 #define ETHERBOOT_ENDIAN_H
 
+FILE_LICENCE ( GPL2_OR_LATER );
+
 /* Definitions for byte order, according to significance of bytes,
    from low addresses to high addresses.  The value is what you get by
    putting '4' in the most significant byte, '3' in the second most