nitscripts: solve the keymap situation in a more generic way to reduce maintenance...

nitscripts: solve the keymap situation in a more generic way to reduce maintenance load
the keymap script is now always installed and checks if there is a matching keymap in /etc
General note: this package is a bit monolithic, we may eventually come to a more fine granular
build solution, e.g. a (build-time) table where distributions can specify which initscripts
they want to appear in the final image.