PHY status API for ethernet drivers

[d-a-v]

Implementation of ::isLinked() for ethernet drivers:
This function returns true when the ethernet chip is physically connected.

(edited) It currently works and has been tested with W5500 and ENC28J60, and it seems it is not implementable with W5100.
An additional ::isLinkDetectable() tells when ::isLinked() is not trivially implemented.

(edited)::connected() is software only and is left unmodified for backward compatibility is updated to check link status to solve compatibility with WiFi API.

Closes #8099

[d-a-v]

@JAndrassy Do you know if Link/PHY status can be read by software for W5100 and ENC28j60 ?

[JAndrassy]

@d-a-v yes it is possible
https://github.com/JAndrassy/EthernetENC/blob/9428b5e970dbc9882ac2aeed94725a1dd87615b9/src/utility/Enc28J60Network.cpp#L629

PHY register 0x11 bit 10
1 = Link is up
0 = Link is down

[d-a-v]

@JAndrassy Thanks !
I successfully used your code on my testboard.

It seems W5100 can't deliver link status readable by software ?
The Arduino driver does not provide it.
I can't seem to find the right register in the datasheet.

[mcspr]

btw do we care about Ethernet compatibility?
https://www.arduino.cc/reference/en/libraries/ethernet/ethernet.linkstatus/
https://github.com/esp8266/Arduino/search?q=linkStatus%28%29

edit: probably not

[d-a-v]

Yes, we do !