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>