Shuffle the dz console code (both serial and keyboard) to be more independent

Shuffle the dz console code (both serial and keyboard) to be more independent
of the console device attachment, and to be able to work on system where there
are more than one dz device of the same type, one of it being the console one.

This also allows a different register layout than the dz@ibus one to be used
for the dz console device.