Bootloader hangs when the ethernet shield is not inserted

[Mioncz]

Is possible for the bootloader to start the program if the W5100 (W5200, ...) shield is not present? Currently, the program does not start if the card is not present, and is not possible to upload the new program via the serial port.

[per1234]

I haven't had time to try it out yet, but here is a proposed solution:
per1234@4888031

Also claimed to be done here:
d0ughb0y@70ff815
but it's mashed together with a lot of other unrelated changes.