manifest: synch MCUBoot to upstream c625da4

MCUBoot was 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>