Turns out this .if evaluated not in the context of the makedtb target

sometimes due to Makefile expansion rules. Make the test for things
being NULL elsewhere...

Makefile.inc1

@@ -1830,9 +1830,6 @@ DTBOUTPUTPATH= ${.CURDIR}
 # Build 'standalone' Device Tree Blob
 #
 builddtb:
-.if !defined(FDT_DTS_FILE)
-.error "FDT_DTS_FILE must be specified!"
-.endif
 	@PATH=${TMPPATH} MACHINE=${TARGET} \
 	${.CURDIR}/sys/tools/fdt/make_dtb.sh ${.CURDIR}/sys \
 	    "${FDT_DTS_FILE}" ${DTBOUTPUTPATH}

sys/tools/fdt/make_dtb.sh

@@ -7,6 +7,11 @@ S=$1
 dts="$2"
 dtb_path=$3
 
+if [ -n "$dts" ]; then
+    echo "No DTS specified"
+    exit 1
+fi
+
 for d in ${dts}; do
     dtb=${dtb_path}/`basename $d .dts`.dtb
     echo "converting $d -> $dtb"