uboot-imx

dtoc: Allow DTC environment variable to provide path to dtc

The system device-tree compiler may not be new enough to run the tests we
use in U-Boot (e.g. with binman). Allow use of a DTC environment variable
to point to the correct dtc. If not defined, the dtc on the default PATH
is used.

Signed-off-by: Simon Glass <sjg@chromium.org>