tests: Add a very basic functional test of the UART

Use the bootloader provided by Atmel in their AVR2054 App Note to
provide a simple functional test of the UART. This also wires that test
up into the makefiles and into Travis so we can have a simple test
validate that drumfish UARTs still work on each commit.
Plumbing to allow us to use libevent

Added build support for libevent as well as initializing libevent.
uart: convert poll to libevent

Rather that use a thread per UART and use blocking I/O for each UART the
code has been switched to an event driven model as supplied by libevent.
This should supply faster responses with less CPU utilization which
means it will behave closer to that of real hardware since it will be
more similar to an interrupt style model.