scripts/buildname.nochroot: adjust datetime substitutions in /etc/tizen-build.conf

scripts/buildname.nochroot: adjust datetime substitutions in /etc/tizen-build.conf

New vars have been introduced by https://review.tizen.org/gerrit/30166.

This patch changes the way that datetime variables are substituted in /etc/tizen-build.conf

* correct the previous code by adding UTC representation
* added substitution for @BUILD_TIME@
* added substitution for @BUILD_TS@

Change-Id: I43ec223019ba6699846effbf6a69c6b5f2b2dc19
Signed-off-by: Stephane Desneux <stephane.desneux@open.eurogiciel.org>