version.sh: Unify version generation

version.sh: Unify version generation

This change includes a few fixes to the version.sh script, in order to
unify the versions generated from git vs. dev- versions. We unify on a
simple YYYYMMDD format, and drop the time specifier (if you're relying
on time info, you probably have the git SHAs to lookup from instead).

We also clean up the date-generation code, by using printf's %T
formatter, on git's %ct time specification, rather than trying to
transform a %ci date.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>