Remove build date from banner for releases

In support for reproducible builds, see
https://wiki.debian.org/ReproducibleBuilds



Fixes Debian bug #834316.

Change-Id: Id81ec72a87bf6dd99abfd2a0ae074658111bc9a3
Signed-off-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Reviewed-on: http://openocd.zylin.com/3866


Tested-by: jenkins
Reviewed-by: Paul Fertser <fercerpav@gmail.com>