fix switching to static address with lwip2
For some reason, ip address is not propagated in a visible way for lwip2
when switching to static address (wifi.config()) *after* wifi.begin().
This patch calls lwip-v1.4's netif_set_addr() with the new ip address to set
all things up, just like it is done and right when wifi.begin() is called
after wifi.config().
Also tested when IPv6 is enabled.
fix documentation: It is more natural to set an IP address before starting WiFi
(.. and not after dhcp has started)
fixes#5839fixes#6024
1750022601
