Add default TCP send buffer configuration

Author: Jason2866

configs/defconfig.common

@@ -94,6 +94,7 @@ CONFIG_LWIP_IPV4_NAPT=y
 # CONFIG_LWIP_DHCP_DOES_ARP_CHECK is not set
 CONFIG_LWIP_TCP_SYNMAXRTX=6
 CONFIG_LWIP_TCP_MSS=1436
+CONFIG_LWIP_TCP_SND_BUF_DEFAULT=5744
 CONFIG_LWIP_TCP_RTO_TIME=3000
 CONFIG_LWIP_TCP_SACK_OUT=y
 CONFIG_LWIP_TCP_MSL=6000