build-{arm-images,rootfs}: added support to build just one platform.

build-rootfs.sh.in

@@ -2,8 +2,12 @@
 
 set -e
 
+TARGET="$1"
+
 PLATFORMS="beaglebone cubieboard2 odroid-u2 rpi rpi2 usbarmory"
 
 for f in ${PLATFORMS}; do
-	./mkrootfs.sh $@ $f
+	if [ -z "$TARGET" -o "$TARGET" = "$f" ]; then
+		./mkrootfs.sh $@ $f
+	fi
 done