Remove dependency with the core

[ghost]

I added an update function to the Ethernet class to remove the dependency inside the main.cpp.
The library is now completely independent of the core.
The function update() updates the LwIP stack also.

[LMESTM]

why do you need this new update interface ? as far as I understood the list of callbacks is a table in core ... so stm32_eth_scheduler will be called directly .... isn't it ?

[fpistm]

Right @LMESTM .
I think @fprwi6labs add it to enhance EthernetClass API that user can use in his sketch.

[LMESTM]

how to use update is not clear to me - please add more information in the commit message why we make the change and how this will/can be used.

[ghost]

It is just in case you go in a blocking loop inside the loop() function, neither updateCoreCallback nor stm32_eth_scheduler will be called. So I just give a possibility to the user to update the LwIP stack to keep Ethernet alive.

[LMESTM]

Ok - I'd like to see this kind of information in the commit message as well.
Typically I guess that you don't want both APIs to be called, user should select one of them ? What is the timing of a loop that requires this API to be called from user application ?
Other point : is update a standard name, or could be consider EthernetClass::schedule() ?

[ghost]

User can use both method.
I don't master enough the LwIP stack to give you a figure.
There isn't a standard name so I can change.

[fpistm]

To avoid any build issue if user do not use the next core version and ensure compatibility, it should be fine to do something like that? Tested and it's ok.

#if __has_include("core_callback.h")
#include "core_callback.h"
#else
void registerCoreCallback(void (*func)(void)) {
  UNUSED(func);
}
#endif