[ESP8266WiFiSTA] Add get broadcast IP address functionality

[a7md0]

Calculate the broadcast IP address using the subnet mask and default gateway IP address.

[devyte]

This looks like generic network calcs. I think it is therefore also applicable to the AP interface, in whichncasenit shouldn't be in the STA class.

[a7md0]

This looks like generic network calcs. I think it is therefore also applicable to the AP interface, in whichncasenit shouldn't be in the STA class.

It would be great if you redirected me to the "generic network class" to add this code.

[d-a-v]

I think what @devyte meant was to move your function into ESP8266WiFiGeneric.cpp with a parameter (the interface index STATION_IF or SOFTAP_IF), and add a call to this function from both ESP8266WiFiSTA and ESP8266WiFiAP with the matching interface index.