I skipped Arduino Uno and work with Raspberry's gpio pins only. No serial communication needed any more ...
The following declares gpio pin 7 to be an input pin:
echo "7" > /sys/class/gpio/export
echo "in" > /sys/class/gpio/gpio7/direction
Whenever I press a button, variable BUTTON gets value 1, otherwise 0
So after booting my radio starts playing with station 1; pressing the button produces a switch to the next station.
When the end of the list is reached, we start again with station 1.