README: Update a bunch of blocks, based on reversing
serial: Use const void * everywhere

Buffers should be stored in read-only areas of memory.
Makefile: Add back in bionic and printf

They really are useful.
main: Add back in command line mode

It's useful to have.
include: Add printf.h
fernvale: Get IRQs to at least do something

IRQs now do something.  They still don't work, though.
irq: Work on getting IRQs working
fernly: Add two-stage bootloader and get basics working

The romulator only emulates 64k of SPI ROM.  That's not enough for anything
interesting.

Add a second-stage bootloader to send data to Fernvale over serial.  This
will allow us to work around needing code relocations, and allow for much
larger binaries.