[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/drivers/net/etherfabric.h

@@ -17,6 +17,8 @@
  **************************************************************************
  */
 
+FILE_LICENCE ( GPL_ANY );
+
 #ifndef EFAB_BITFIELD_H
 #define EFAB_BITFIELD_H