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>

Makefile

@@ -1280,10 +1280,16 @@ define filechk_version.h
 endef
 
 define filechk_timestamp.h
-	(SOURCE_DATE="$${SOURCE_DATE_EPOCH:+@$$SOURCE_DATE_EPOCH}"; \
-	LC_ALL=C date -u -d "$${SOURCE_DATE:-now}" +'#define U_BOOT_DATE "%b %d %C%y"'; \
-	LC_ALL=C date -u -d "$${SOURCE_DATE:-now}" +'#define U_BOOT_TIME "%T"'; \
-	LC_ALL=C date -u -d "$${SOURCE_DATE:-now}" +'#define U_BOOT_TZ "%z"' )
+	(if test -n "$${SOURCE_DATE_EPOCH}"; then \
+		SOURCE_DATE="@$${SOURCE_DATE_EPOCH}"; \
+		LC_ALL=C date -u -d "$${SOURCE_DATE}" +'#define U_BOOT_DATE "%b %d %C%y"'; \
+		LC_ALL=C date -u -d "$${SOURCE_DATE}" +'#define U_BOOT_TIME "%T"'; \
+		LC_ALL=C date -u -d "$${SOURCE_DATE}" +'#define U_BOOT_TZ "%z"'; \
+	else \
+		LC_ALL=C date +'#define U_BOOT_DATE "%b %d %C%y"'; \
+		LC_ALL=C date +'#define U_BOOT_TIME "%T"'; \
+		LC_ALL=C date +'#define U_BOOT_TZ "%z"'; \
+	fi)
 endef
 
 $(version_h): include/config/uboot.release FORCE