Makefile: Use correct timezone for U_BOOT_TZ

When building with SOURCE_DATE_EPOCH the timezone is in UTC. When
building normally the timezone is taken from the build machine's locale
setting.

Signed-off-by: Chris Packham <judge.packham@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Paul Kocialkowski <contact@paulk.fr>
Tested-by: Andreas Bießmann <andreas.devel@googlemail.com>