manifest: mcuboot update

Limit allowed build platforms.
Building sample.bootloader.mcuboot for many platforms
is not possible (for instance a qemu).
The limit is need as otherwise zephyr-rtos/zephyr CI is
failing on any push to main branch or nightly CI run.

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