DEMO: Add demo program from https://github.com/libopencm3/libopencm3-examples/tree/master/examples/stm32/f4/stm32f4-discovery/button
DEMO: Add demo program from https://github.com/libopencm3/libopencm3-examples/tree/master/examples/stm32/f4/stm32f4-discovery/button

fnivek/Pop-a-Gator,fnivek/Pop-a-Gator,fnivek/Pop-a-Gator