Default USB Interface backend

[cederom]

Hey @flit :-)

I have noticed that a lot of internals changed, also the USB backend handling, we can discuss proposed major release number bump here https://github.com/pyocd/pyOCD/issues/1155.

The initial problem report on USB backend problems were repoted here: https://github.com/pyocd/pyOCD/issues/1032, https://github.com/pyocd/pyOCD/issues/1144.

I have added BSD detection support here https://github.com/pyocd/pyOCD/pull/1153 and modified default error message here https://github.com/pyocd/pyOCD/pull/1154.

I would like to propose pyusb as the default fallback on platforms that are not on the list. That should bring support to USB Interfaces on "unknown" platforms with a backend that is known to be most versatile and platform independent. If pyusb is not present or cannot be used on an unknown platform we will know that directly.

My proposition can be found here https://github.com/pyocd/pyOCD/pull/1156 and is open to discussion :-) We have to remove commented out raise line and maybe replace it with some sort of warning instead :-)