net: tcp: TCP transmission

Hook up TCP transmission through the net_context_send() API.  Queues
packets via a list in the net_buf header, but as of right now simply
transmits the queue synchronously, ignoring the need for retransmit
and the limits of the receive window on the other side.

Requires that the ACK transmission be moved ahead of the net_context
callback invokation.  This to work around a glitch in the way ACKs
work with queueing (they depend on current state, but packets are
assembled just once) that will be fixed in a coming patch.

Change-Id: I7490333e4b314e7734fcc03f2a63d76ae89d698a
Signed-off-by: Andy Ross <andrew.j.ross@intel.com>
Signed-off-by: Leandro Pereira <leandro.pereira@intel.com>