[yocto] File system for alternate partition

[Bollinger, Seth]

>You could just delete these within a function added to
>ROOTFS_POSTPROCESS_COMMAND, and that's probably the way to deal with it
>for 
>now; however for the future I do think the build system ought to be able
>to 
>clean these files up itself if they aren't supposed to be there. I've
>created a 
>bug to track the issue:
>
>https://bugzilla.yoctoproject.org/show_bug.cgi?id=6478

It looks like the POSTPROCESS commands are run too early.  Subsequent
actions depend on that directory structure (pre-linking, etc.).  It looks
like I would need some way to hook _cleanup() in the RootFS subclass.

Seth