Build raw image in separate tmpfs

If the tmpfs being used to hold the image filesystem is close to being
full then housing the raw disk image in the same place can be
problematic as it tends to grow faster then the filesystem shrinks when
data is being moved into it.

Putting them both into separate tmpfs's will allow there to be an
overlap where they jointly use more then the size limit for a singe tmpfs.

Change-Id: Ia17ca357d3b865d70a2d3e13e0479d008ca5f924
Closes-Bug: #1289582
Merge "Build raw image in separate tmpfs"