boot/uboot: set DTC path when BR2_TARGET_UBOOT_NEEDS_DTC
In order for host-dtc to get picked up properly we need to set
the DTC path passed to the build explicitly.
See:
https://github.com/u-boot/u-boot/blob/v2023.07.02/Makefile#L420
Signed-off-by: James Hilliard <james.hilliard1@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>