Keyboard driver stub

Begin a keyboard driver (with data for a german keyboard). The current one
can print upper and lower case characters to screen. It is badly written and
has some bugs in the data (not talking about non-existent data). I just
don't want to leave the tree un-committed over the night :-)

At least, the keyboard can be initialised and the ISR registered with this
commit.

Methinks, a %c conversion should also be added to printf().