zephyr/sample.yaml 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>