Exclude /tmp/images from cleanup

/tmp is deleted every 10 days
/usr/lib/tmpfiles.d/tmp.conf does this cleanup with this entry,
"q /tmp 1777 root root 10d".
Exclude /tmp/images from the cleanup, by adding "x /tmp/images"
to /usr/lib/tmpfiles.d/software.conf, if a feature of the machine is
obmc-ubi-fs or openpower-ubi-fs.

Resolves openbmc/openbmc#2544

Change-Id: I3ead88724adcd63ebfd809d310109a19232b1def
Signed-off-by: Gunnar Mills <gmills@us.ibm.com>