net: Fix enabling both Bluetooth and Networking support

We can't use '=' in net/Makefile since then one statement would
override the value of the previous one when both CONFIG_NETWORKING and
CONFIG_BLUETOOTH were set to 'y'. Instead, use '+='.

Change-Id: Idba9916cc9fb2bd0e53975bdf0a86c0fd184533c
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>