net-caif-driver: add CAIF serial driver (ldisc)

Add CAIF Serial driver. This driver is implemented as a line discipline.

caif_serial uses the following module parameters:
ser_use_stx - specifies if STart of frame eXtension is in use.
ser_loop    - sets the interface in loopback mode.

Signed-off-by: Sjur Braendeland <sjur.brandeland@stericsson.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/drivers/net/Kconfig b/drivers/net/Kconfig
index 4f1f974..da6657c 100644
--- a/drivers/net/Kconfig
+++ b/drivers/net/Kconfig
@@ -2837,6 +2837,8 @@
 
 source "drivers/s390/net/Kconfig"
 
+source "drivers/net/caif/Kconfig"
+
 config XEN_NETDEV_FRONTEND
 	tristate "Xen network device frontend driver"
 	depends on XEN