boards/chronos: clean up display driver a bit

reduce scope of variables
remove useless comments and spaces

saves 2 bytes =)
native: increase rtc realism

Prevent rtc enabling when not initialized.
Merge pull request #1474 from LudwigOrtmann/chronos_cleanup_display

boards/chronos: clean up display driver a bit
Merge pull request #1477 from LudwigOrtmann/native_rtc_increase_realism

native: increase rtc realism