repart: perform an early check for missing mkfs or fs contents

repart: perform an early check for missing mkfs or fs contents

I was running repart in a VM, and if failed because mkfs.vfat was
not available. But if fails quite late in the process, possibly wasting
quite a bit of work. So add a check that catches some obvious cases
where repart would fail.

The condition of whether we have the root directory is complex,
determined in part by partition_target_prepare(). I didn't think it
was worth it to recreate the full logic in the check, so in some cases
it'll not miss cases. But that's still better than having no check ;)