Only create a tmpfs big enough for DIB_MIN_TMPFS

This will help to ensure images that would cause bug #1289582 would
still fail even on systems that have more RAM. Also this will ensure we
only reserve enough tmpfs space for this build.

Change-Id: Icece64e691d126a5fc89262897630fdbef1ffdf1
Merge "Only create a tmpfs big enough for DIB_MIN_TMPFS"