net: tcp: add timeout wait in net_context_connect

If the parameter "timeout" is set in net_context_connect(), the
assumption by the user is that the function would wait for SYNACK
to be received before returning to the caller.

Currently this is not the case.  The timeout parameter is handed
off to net_l2_offload_ip_connect() if CONFIG_NET_L2_OFFLOAD_IP is
defined but never handled in a normal call.

To implement the timeout, let's use a semaphore to wait for
tcp_synack_received() to get a SYNACK before returning from
net_context_connect().

Change-Id: I7565550ed5545e6410b2d99c429367c1fb539970
Signed-off-by: Michael Scott <michael.scott@linaro.org>