Text mode, still using DMA.

Here, instead of using a frame buffer, DMA alternates between
two line buffers. While one is being output, the other is being
prepared from a text mode screen and a text font.

An alternative would be to directly load SSI from interrupts
instead of using a buffer plus DMA. This was done because it was
a simpler change. Performance implications need to be
investigated.
Add polled UART code. There is no terminal emulation yet.
Add terminal emulation using ROTE.

This adds a stripped down version of the ROTE library from
http://rote.sourceforge.net/. It is used to emulate a terminal
for data received from the serial port. It works, though there
might be overruns in some situations.