synch with upstream c625da41

Synchronized up to:
https://github.com/JuulLabs-OSS/mcuboot/commit/c625da4



- Removed the `flash_area_read_is_empty()` port implementation function
- Added watchdog feed on nRF dvices. See CONFIG BOOT_WATCHDOG_FEED option.

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