Commit c90bd74ebb ("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
c90bd74ebb.
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>