[CA-141724] Modify --signcmd to accept both a string for a signing
command and a string for a cross-signing command

Signed-off-by: Ben Chalmers <Ben.Chalmers@citrix.com>
Merge pull request #75 from benchalmers/ca141724

[CA-141724] add a cross-sign command to the --signcmd argument

Approved-By: ben.chalmers@citrix.com