A few fixes and configuration + sync button support

Fixes/configuration:

  * configure switches debounce time
  * reduce chip frequency since high speed is not needed
  * send initial footswitches state at power on
  * add "UL" to big constants
  * make "g_midi_status_byte" a constant
  * improve comments

Sync button support means an optional (see "config.h")  button
may be added to the design to resend all MIDI cc values for
current footswitches states. This is needed to synchronize the
footswitches values with the actual virtual values we are
controlling.