Support %c in printf()

This is kind of helpful for testing the keyboard driver.

Well, I wasn't that productive today because I decided to begin backing up
my hard drive. The only thing was that I used rsync with --delete and
erroneously exchanged source and destination operand. Yeah, that was my
afternoon and evening: desperately trying to get the lost data back somehow
and by the way setting up the mess that calls itself systemd. I really
appreciated sysvinit and I think I'm going to ultimately leave what was once
Arch Linux - or what once used to be KISS.