Fixes wrong date format in frankenmerged manifest.

By now, the value of the DATE field in the manifest header was set to be
equal to the faketime timestamp we use for the build. However, as it
turned out with the rollout of 0.6.0~rc2, the Gluon autoupdater relies
on a slightly different command and thus rejected the frankenmerged
manifest file. By "different" I mean that it is identical up to the last
character, but requires a suffix which indicates the UTC offset. E.g.
when the faketime timestamp is "2015-02-14 23:42:00", the correct value
in the DATE field looks like "2015-02-14 23:43:00+01:00", Hence, the
issue gets fixed by appending a simple "$(date +%:z)" to the TS variable.

build.sh

@@ -169,7 +169,7 @@ for target in ${TARGETS} ; do
 done
 frankenmerge_manifest_file="./images/sysupgrade/${BRANCH}.manifest"
 echo "BRANCH=${BRANCH}" > ${frankenmerge_manifest_file}
-echo "DATE=${BUILD_TS}" >> ${frankenmerge_manifest_file}
+echo "DATE=${BUILD_TS}$(date +%:z)" >> ${frankenmerge_manifest_file}
 echo "PRIORITY=0" >> ${frankenmerge_manifest_file}
 echo "" >> ${frankenmerge_manifest_file}
 grep -hE "*.bin$" ./images/sysupgrade/*.${BRANCH}.manifest | sort >> ${frankenmerge_manifest_file}