Fix signature-build under OSX

On OS X when you use -i an extension for the in-place
substitution a backup files is required.

http://stackoverflow.com/questions/4247068/sed-command-failing-on-mac-but-works-on-linux

So rewrite to make sed flushing sign.txt in one go and avoid
using -i option.

Reported by Louis Zulli

No functional change.