cc2538dk: Make it possible to override UIP_CONF_TCP_MSS
cc2538dk: Make it possible to override UIP_CONF_TCP_MSS

Signed-off-by: Benoît Thébaudeau <835bc8b01b82c0117127b72626b289641d7633e2@advansee.com>

arurke/contiki,MohamedSeliem/contiki,MohamedSeliem/contiki,arurke/contiki,arurke/contiki,MohamedSeliem/contiki,MohamedSeliem/contiki,arurke/contiki,arurke/contiki,MohamedSeliem/contiki,MohamedSeliem/contiki,arurke/contiki,arurke/contiki,bluerover/6lbr,bluerover/6lbr,MohamedSeliem/contiki,bluerover/6lbr,bluerover/6lbr,bluerover/6lbr,bluerover/6lbr,bluerover/6lbr