1. Modify dhcp server timer to 1 seconds 2. Enable ETHARP_TRUST_IP_MAC modify according to review
LWIP will drop the oldest package at the arp packet cache queue and return "ERR_OK" We change it that ARP level will drop the current packet and return "ERR_MEM" to tell user that you should send the packet again latter.
1. All espressif specific code are prefix with ESP_ 2. Define all ESP_ options in lwipopts.h 3. Remove useless code added in 8266
