Brymen BM869 support

Yes, it's time for another multimeter! This time libsigrok has gained support for the Brymen BM869 multimeter (the code was written by Aurelien Jacobs, thanks a lot!).

This is a 50000 counts True RMS DMM with various "higher-end" features compared to other, cheaper DMMs.

The PC connectivity interface Brymen BU-86X is using USB/HID internally (based on the Cypress CY7C63743 enCoRe USB chip), the protocol is relatively simple and documented by the manufacturer.

You can also browse the source code if you're interested in some of the implementation details.