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>

tools/mkimage/Makefile

@@ -30,6 +30,7 @@ define Host/Compile
 		HOSTCFLAGS="$(HOST_CFLAGS)" \
 		HOSTLDFLAGS="$(HOST_LDFLAGS)" \
 		no-dot-config-targets=tools-only \
+		CONFIG_MKIMAGE_DTC_PATH=dtc \
 		CONFIG_FIT_SIGNATURE=y \
 		tools-only
 endef