Allow rvdb to receive characters from the Otter over serial

[hrinn]

This PR allows rvdb to receive and print messages from the Otter. To print messages from the Otter, you will need to implement the AXI mhub with the serial controller that I am working on (and haven't finished yet).

In the CLI, it polls the stdin fd and the serial port fd and reacts based on which one received input. Because it hangs before the user enters input, the readline library no longer looks good. I replaced it with fgets, but that means the command history and auto completion no longer works. Because these features are gone, and since this is meant to work with a specific version of the Otter, I would recommend keeping this on a separate branch.

[hrinn]

[hrinn]

Here is the repo for my project, its finished and public now: