6lowpan: Allow 6LoWPAN to be modular

Change config symbol 6LOWPAN from type bool to type tristate, so
6LoWPAN can be built modular, just like IPV6

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: David S. Miller <davem@davemloft.net>