tools: mkimage: provide dtc path during build

tools: mkimage: provide dtc path during build

The new mkimage version requires a CONFIG_MKIMAGE_DTC_PATH variable to be
provided during build, in order to hardcode a path to a suitable DT
compiler executable.

Failure to do so will result in stray "sh: 1: -I: not found" errors when
invoking mkimage for FIT image generation.

Fix the issue by supplying "dtc" as CONFIG_MKIMAGE_DTC_PATH value during
build. As we intend our host utilities to be relocatable and since we're
already overriding PATH when invoking mkimage, an absolute path is not
required.

Fixes: b13e981d72 ("tools/mkimage: update to version 2018.03")
Signed-off-by: Jo-Philipp Wich <jo@mein.io>