boot/barebox: explicitly set the timezone for the kbuild timestamp

This adjusts the code added in commit
c90bd74ebb264ccc925f10a142ccd82ef63a4601 ("boot/barebox: pass required
environment variables for reproducible build") to explicitly set the
timezone when calculating KBUILD_BUILD_TIMESTAMP, like is already done
in linux/linux.mk.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
(cherry picked from commit 11e75ecbf092420f00f0fc88a7183d5d99887ff1)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

boot/barebox/barebox.mk

@@ -78,7 +78,7 @@ ifeq ($$(BR2_REPRODUCIBLE),y)
 $(1)_MAKE_ENV += \
 	KBUILD_BUILD_USER=buildroot \
 	KBUILD_BUILD_HOST=buildroot \
-	KBUILD_BUILD_TIMESTAMP="$$(shell LC_ALL=C date -d @$(SOURCE_DATE_EPOCH))"
+	KBUILD_BUILD_TIMESTAMP="$$(shell LC_ALL=C TZ='UTC' date -d @$(SOURCE_DATE_EPOCH))"
 endif
 
 ifeq ($$(BR2_TARGET_$(1)_USE_DEFCONFIG),y)