uboot: Prevent overwriting of build date

After changing the distro config to allow reproducible builds, a side
effect was that the date printed during uboot startup was 1 Jan 1970.

In order to get the real build date in uboot, we must unset the
SOURCE_DATE_EPOCH variable.

BugzID: 67738

recipes-bsp/u-boot/u-boot-nm.inc

@@ -64,6 +64,10 @@ UBOOT_ENV_BINARY ?= "${UBOOT_ENV}.${UBOOT_ENV_SUFFIX}"
 UBOOT_ENV_IMAGE ?= "${UBOOT_ENV}-${MACHINE}-${PV}-${PR}.${UBOOT_ENV_SUFFIX}"
 UBOOT_ENV_SYMLINK ?= "${UBOOT_ENV}-${MACHINE}.${UBOOT_ENV_SUFFIX}"
 
+# This variable is set to "0" in distro in order to get reproducible build
+# It is however not needed for uboot and unsetting it allows us to keep the build date in uboot output
+unset SOURCE_DATE_EPOCH
+
 do_compile () {
     if [ "${@bb.utils.contains('DISTRO_FEATURES', 'ld-is-gold', 'ld-is-gold', '', d)}" = "ld-is-gold" ] ; then
         sed -i 's/$(CROSS_COMPILE)ld$/$(CROSS_COMPILE)ld.bfd/g' config.mk