Convert the sample to use the new GPIO API and additionally:
- add some error messages for unsuccessful GPIO API calls
- correct the index of `def_val` element used in the `right_button`
callback, to match the one used when the callback is installed
- use flags defined in devicetree for the pin that drives the LED
(for consistency, as this does not make much difference for a pin
that is only toggled)
Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>
0c10d5cc1f