Improve logging

[misson20000]

Blocked on #47.

Currently, every call to printf in Twili blocks on a USB transfer. There should be several different logging modes:

• USB Synchronous: Each call to printf blocks until the data is received by twibd. Useful for debugging.
• USB Asynchronous: When printf is called, if we are waiting on a USB transfer, queue the message and submit another transfer when that one completes. Otherwise, submit a new transfer and don't want for it to complete. Should be more performant than synchronous.
• Transfer: printf appends to a ringbuffer that can be read out via a bridge request. Useful for when USB logging isn't an option, or when it's not important that logs be transferred out-of-band. This mode should never block.