disk/part: Only build hostfs special handling when CONFIG_SANDBOX is set

This is not necessary / useful when not building with CONFIG_SANDBOX and
with the addition of ubifs support to the generic fs commands it actually
gets in the way, since both operate on a fake / NULL blkdev.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Acked-by: Stephen Warren <swarren@nvidia.com>