boot/barebox: use $$ instead of $ where needed

Commit c90bd74ebb264ccc925f10a142ccd82ef63a4601 ("boot/barebox: pass
required environment variables for reproducible build") caused the
build to generate a spurious error message due to the $(shell
... date -d @$(SOURCE_DATE_EPOCH)) command being executed even when
BR2_REPRODUCIBLE was disabled.

This is due to the fact that variable references must be done using $$
within macros. This commit fixes that for the two variables that we
are referencing in the code added in commit
c90bd74ebb264ccc925f10a142ccd82ef63a4601.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

boot/barebox/barebox.mk

@@ -74,11 +74,11 @@ endif
 $(1)_MAKE_FLAGS = ARCH=$$($(1)_ARCH) CROSS_COMPILE="$$(TARGET_CROSS)"
 $(1)_MAKE_ENV = $$(TARGET_MAKE_ENV)
 
-ifeq ($(BR2_REPRODUCIBLE),y)
+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 date -d @$(SOURCE_DATE_EPOCH))"
 endif
 
 ifeq ($$(BR2_TARGET_$(1)_USE_DEFCONFIG),y)