make_dtb.sh is designed to be used in a kernel build environment where

MACHINE is defined to the target's value, not the host's
value. However, in Makefile.inc1, it is still defined to be the host's
value. Make the makedtb target work by expanding TARGET in the
existance test, and passing MACHINE=$TARGET in the call to make_dtb.sh

Makefile.inc1

@@ -1833,7 +1833,7 @@ builddtb:
 		echo "ERROR: FDT_DTS_FILE must be specified!"; \
 		exit 1; \
 	fi;	\
-	if [ ! -f ${.CURDIR}/sys/boot/fdt/dts/${MACHINE}/${FDT_DTS_FILE} ]; then \
+	if [ ! -f ${.CURDIR}/sys/boot/fdt/dts/${TARGET}/${FDT_DTS_FILE} ]; then \
 		echo "ERROR: Specified DTS file (${FDT_DTS_FILE}) does not \
 			exist!"; \
 		exit 1;	\
@@ -1843,6 +1843,7 @@ builddtb:
 			directory"; \
 	fi
 	@PATH=${TMPPATH} \
+	MACHINE=${TARGET} \
 	${.CURDIR}/sys/tools/fdt/make_dtb.sh ${.CURDIR}/sys \
 	    ${FDT_DTS_FILE} \
 	    ${DTBOUTPUTPATH}/`basename ${FDT_DTS_FILE} .dts`