net-caif: add CAIF Kconfig and Makefiles

Kconfig and Makefiles with options for:
CAIF:        Including caif
CAIF_DEBUG:  CAIF Debug
CAIF_NETDEV: CAIF Network Device for GPRS Contexts

Signed-off-by: Sjur Braendeland <sjur.brandeland@stericsson.com>
Signed-off-by: David S. Miller <davem@davemloft.net>