Don't bundle flashing into functional tests fab task, give me control.
Grab correct packet size for incoming EP0 USB transfers on PIC32.

Fixes most if not all control commands. Caught by new functional tests,
yay!
Allow device ID to be "Unknown" when testing a VI without BT.
Remove a redundant reset of CAN AF when changing on the fly.

It's already set by updateAcceptanceFilterTable after configuring the
filters as required.
Don't hold filter objects for buses where the AF is disabled.

This fixes a bug where the count of the total number of acceptance
filters is off, and the CAN library gets confused when configuring the
second bus.

The functional test suite caught this, specifically
CanMessageTests.test_matching_can_message_received_on_filtered_bus.

Fixed #260.
Remove a stray debugging printout.
Don't use docstrings in nosetests, it obscures the test name in output.
Reset AF status before each func. test to get a stable state.