Enable the user to specify the serial number

[hmaarrfk]

This enables the user to specify the serial to program.

Works with libusb, not sure about windows.

[hmaarrfk]

For windows, it seems we need to use :
https://docs.microsoft.com/en-us/windows-hardware/drivers/ddi/hidsdi/nf-hidsdi-hidd_getserialnumberstring

I don't have a handy windows machine.

[hmaarrfk]

@PaulStoffregen is there any interest in enabling this feature?

If so, I can work slowly on improving feature parity between different operating systems.

[botto]

I'm quite interested in this feature. Is it stable on Linux.
I do have a windows machine so I can give it a go.

[hmaarrfk]

I has been working for me.

[FiratSusan]

#56 (comment)

Not sure but I should have posted it here...

[hmaarrfk]

@PaulStoffregen is there any interest in integrating such a feature?

[PaulStoffregen]

Maybe, but let me say very clearly that no new features are likely to be merged until Oregon relaxes Covid19 social distancing requirements and PJRC can rehire.

[velovix]

Would it be possible to get some eyes on this pull request now that things have calmed down a bit? I have a single industrial PC that connects to ~5 Teensys in an enclosure, so it would be very convenient to not have to physically press their flash buttons on each iteration.