boot/barebox: pass required environment variables for reproducible build

Barebox makes use of the same variables as the linux kernel does for
handling reproducible build -- KBUILD_BUILD_HOST,
KBUILD_BUILD_TIMESTAMP, KBUILD_BUILD_USER. This patch sets the proper
variables based on linux/linux.mk, and passes them to the make
invocation when building, to ensure a reproducible build is possible
when BR2_REPRODUCIBLE is enabled.

Signed-off-by: Casey Reeves <casey@xogium.me>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
(cherry picked from commit c90bd74ebb264ccc925f10a142ccd82ef63a4601)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

boot/barebox/barebox.mk

@@ -74,6 +74,13 @@ endif
 $(1)_MAKE_FLAGS = ARCH=$$($(1)_ARCH) CROSS_COMPILE="$$(TARGET_CROSS)"
 $(1)_MAKE_ENV = $$(TARGET_MAKE_ENV)
 
+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))"
+endif
+
 ifeq ($$(BR2_TARGET_$(1)_USE_DEFCONFIG),y)
 $(1)_KCONFIG_DEFCONFIG = $$(call qstrip,$$(BR2_TARGET_$(1)_BOARD_DEFCONFIG))_defconfig
 else ifeq ($$(BR2_TARGET_$(1)_USE_CUSTOM_CONFIG),y)
@@ -121,7 +128,7 @@ endef
 
 define $(1)_BUILD_CMDS
 	$$($(1)_BUILD_BAREBOXENV_CMDS)
-	$$(TARGET_MAKE_ENV) $$(MAKE) $$($(1)_MAKE_FLAGS) -C $$(@D)
+	$$($(1)_MAKE_ENV) $$(MAKE) $$($(1)_MAKE_FLAGS) -C $$(@D)
 	$$($(1)_BUILD_CUSTOM_ENV)
 endef