Use single characters for triggering updates via serial

This commit removes string buffering and simply settles for a single
character interface over the serial line with the following mapping:

  's': set status to success
  'r': set status to running
  'f': set status to failed

All other characters are ignored.
Add triggering status update via buttons

This commit let's the user trigger status update via the gamekit buttons
using the following mapping:

  UP:            set status to success
  LEFT or RIGHT: set status to running
  DOWN:          set status to failed
Properly handle C++ enum type
Don't use images for display status
Use custom dimming pixel function for status running