Bluetooth: make bluetooth 6lowpan as an option

Currently you can have bluetooth 6lowpan without ipv6 enabled. This
doesn't make any sense. With this patch you can disable/enable bluetooth
6lowpan support at compile time.

The current bluetooth 6lowpan implementation doesn't check the return
value of 6lowpan function. Nevertheless I added -EOPNOTSUPP as return value
if 6lowpan bluetooth is disabled.

Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>

net/bluetooth/Kconfig

@@ -12,7 +12,6 @@ menuconfig BT
 	select CRYPTO_AES
 	select CRYPTO_ECB
 	select CRYPTO_SHA256
-	select 6LOWPAN_IPHC
 	help
 	  Bluetooth is low-cost, low-power, short-range wireless technology.
 	  It was designed as a replacement for cables and other short-range
@@ -40,6 +39,13 @@ menuconfig BT
 	  to Bluetooth kernel modules are provided in the BlueZ packages.  For
 	  more information, see <http://www.bluez.org/>.
 
+config BT_6LOWPAN
+	bool "Bluetooth 6LoWPAN support"
+	depends on BT && IPV6
+	select 6LOWPAN_IPHC
+	help
+	  IPv6 compression over Bluetooth.
+
 source "net/bluetooth/rfcomm/Kconfig"
 
 source "net/bluetooth/bnep/Kconfig"