Merge crctool updates from Sourcebots

[kierdavis]

The SourceBots fork of this repository has a few improvements, mainly to crctool: https://github.com/srobo/dfu-bootloader/commits/ce04f2f241ee25d9e7c3348d48f016d7d0cf8d48

It might be worth merging them into this repository.

[WillB97]

These improvements should be included in crctool.py in #6 (now in python).