Changes to allow configuration on a system with multiple FTDI devices

[chrisys]

No description provided.

[chrisys]

@kersing is there any reason why this can't be merged?

[kersing]

Didn’t see a notification so I wasn’t aware there was a pull request.

Requests:

1. Make the index configurable with a sane default if not defined. Preferably by reading from an environment variable. Check libloragw/src/loragw_spi.native.c SPI_SPEED for an example.
2. Please don’t submit patches with whitespace changes, now I have to check many lines that aren’t real changes. If you want to remove whitespace, make it a separate commit that I can glance over.

[chrisys]

@kersing I've addressed those points - let me know if that's acceptable now.