[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/include/bits/umalloc.h

@@ -7,6 +7,8 @@
  *
  */
 
+FILE_LICENCE ( GPL2_OR_LATER );
+
 #include <gpxe/memtop_umalloc.h>
 
 #endif /* _BITS_UMALLOC_H */