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>

net/6lowpan/Kconfig

 config 6LOWPAN
-	bool "6LoWPAN Support"
+	tristate "6LoWPAN Support"
 	depends on IPV6
 	---help---
 	  This enables IPv6 over Low power Wireless Personal Area Network -