Allow serial upload if W5x00 is not attached

[per1234]

If Ethernet controller is not detected then still allow serial upload and use a shorter bootloader delay. This is claimed to have been done already in the NetLoad bootloader.

Update the Troubleshooting section of the documentation.

Related: loathingKernel#30

[hagaigold]

update[2017-06-19] - rebase of code, so commit SHA chnaged

I done it in a bit different way: the decision is base on initialize failure in sockInit() function.
please see 4888031

also i think it's should titled as a bug and not as an enhancement. reason is that serial upload is part of that boot-loader, and it fail to work on some conditions.

[per1234]

Thanks! I'm very excited for this. I think the serial upload only working with the Ethernet controller connected was a very common source of confusion for people. It's even gotten me a couple of times and I should have known better. It's going to take me a little while to get the spare time to test this out but I'll definitely merge it as soon as possible.