uboot-tools: fix dtc invocation

uboot-tools: fix dtc invocation

Since uboot 2017.09 path to dtc will be configured via Kconfig. As BR
skips this step for uboot-tools building one has to provide
CONFIG_MKIMAGE_DTC_PATH=dtc on the build command line. Otherwise
mkimage will not be able to create FIT images, i.e.:

mkimage -f kernel-fit.its kernel-fit.itb

will fail with very weird errors.

Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>