grml-lang: use /etc/default/keyboard instead of /etc/sysconfig/keyboard

For proper font handling we switched to usage of console-setup and
keyboard-configuration packages. We adjusted grml-live and
grml-autoconfig to use /etc/default/keyboard instead of
/etc/sysconfig/keyboard. To better handle this also within grml-lang
itself, switch to usage of /etc/default/keyboard.

FTR, this is not the final and bullet-proof solution yet, but meant to
be fix the main issues (e.g. booting with Grml defaults, then pressing
'd' for german keyboard within grml-quickconfig and then starting X via
'x' within grml-quickoconfig), using a minimal approach.

Initial work addressing https://github.com/grml/grml/issues/115