Add -s option to get just the sig file for an image (use with -r). Also fix bug in how imagevalidate is invoked, which resulted in the hash being computed for the previous version of the image, not the new version.
Add -s option to get just the sig file for an image (use with -r).
Also fix bug in how imagevalidate is invoked, which resulted in the
hash being computed for the previous version of the image, not the
new version.

nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome,nmc-probe/emulab-nome