manifest: add reference to mcuboot project

MCUBoot is the bootloader on which zephyr DFU solutions
base. It is worth to reference certain compatible version
of this external project. So fare it was expected that
mcuboot works with zephyr in master to master relation. This patch
starts to give the user real information about compatible version.

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>
Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>