In general is recommended to have a separate /boot, but not strictly
required.

In some cases it is required but we need to add API for that, because it
depends mainly on the filesystem of the root partition.

So now the requirement regards mainly biosboot and efi partitions, which
is dependent on the platform.

TODO: update tests

Instead of adding GetRecommendedMountPoints to GetRequiredMountpoints
replace the both with a bit more generic GetMountPointConstraints
that would add recommended/required flag to the constraints.