Merge branch 'lwip/optimize_dhcpc_taken_time' into 'master'

lwip: optimize the dhcp client

1. modify the discover retry backoff time from (2,4,8,16,32,60,60)s to (500m,1,2,4,8,15,15)s.
2. add DHCP_DOES_ARP_CHECK to menuconfig for users to specify if do a ARP check on the offered address.
   If enable, one more second will be taken.

See merge request !459