zephyr/main: configure DFU detection GPIO-pin uisng DTS

Added support for configure GPIO-pin used for triggering the
recovery DFU using DTS. That pin is assigned thanks to
`mcuboot-button0` pin-allias.

DTS configuration takes precedence over the Kconfig pin configurations.

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>