initramfs: allow setting the MAC address from a file in the sd card

[EmilioPeJu]

No description provided.

[EmilioPeJu]

I think I like the idea of deleting the MAC configuration logic from the initramfs script, the ideal place for doing that in my opinion would be in the panda config service (which configures the network) and it would base the configuration on target-defs (as Tom suggested), however, there is a limitation there, a systemd service can never prompt the user for input.... so here is the question: Could we get rid of this feature of asking for the MAC and just support the MAC file in the SD card for that purpose?
My proposal is doing it in the get_mac_address function in target-defs, something like:

get_mac_address()
{
    [ -f /boot/MAC ] && cp /boot/MAC /qspi/MAC && mv /boot/MAC /boot/MAC.done
    cat /qspi/MAC
}

no need to validate MAC, the ifconfig command will do it. This also has the extra feature that it will not fail the boot if the MAC address is not set, I think that is better because at least you can fix the problem using ssh

[EmilioPeJu]

It was decided to keep the same read MAC logic in initramfs init script