Added method to framer that returns the header length if the framer

would create a header with the current packet information.

This allows sicslowpan to calculate the max payload size without
consuming a sequence number or clearing/restoring the packet buffer.

core/net/mac/framer.h

@@ -45,6 +45,7 @@
 
 struct framer {
 
+  int (* length)(void);
   int (* create)(void);
   int (* parse)(void);