tools/mkimage: Make the path to the dtc binary that mkimage calls configurable

In some cases, such as FreeBSD, the path to an alternative dtc needs to
be used.  Rather than override the one given in the Makefile on the
command line, make this part of the build configuration.

Signed-off-by: Tom Rini <trini@konsulko.com>

dts/Kconfig

@@ -170,3 +170,12 @@ config TPL_OF_PLATDATA
 	  information.
 
 endmenu
+
+config MKIMAGE_DTC_PATH
+	string "Path to dtc binary for use within mkimage"
+	default "dtc"
+	help
+	  The mkimage host tool will, in order to generate FIT images make
+	  calls to the dtc application in order to create the output.  In
+	  some cases the system dtc may not support all required features
+	  and the path to a different version should be given here.

tools/Makefile

@@ -175,7 +175,7 @@ HOSTCFLAGS_rsa-sign.o += -Wno-deprecated-declarations
 endif
 endif
 
-HOSTCFLAGS_fit_image.o += -DMKIMAGE_DTC=\"$(DTC)\"
+HOSTCFLAGS_fit_image.o += -DMKIMAGE_DTC=\"$(CONFIG_MKIMAGE_DTC_PATH)\"
 
 HOSTLOADLIBES_dumpimage := $(HOSTLOADLIBES_mkimage)
 HOSTLOADLIBES_fit_info := $(HOSTLOADLIBES_mkimage)