scripts/mkimg.netboot.sh: create symlink to directory

create a symlink to latest netboot release. This is to make it easy to
create urls without version numbers like:
http://dl-cdn.alpinelinux.org/alpine/latest-release/releases/x86_64/netboot

scripts/mkimg.netboot.sh

 create_image_netboot() {
-	rm -rf "${OUTDIR}"/netboot-$RELEASE
+	rm -rf "${OUTDIR}"/netboot-$RELEASE "${OUTDIR}"/netboot
 	cp -aL "${DESTDIR}"/boot "${OUTDIR}"/netboot-$RELEASE
+	ln -s netboot-$RELEASE "${OUTDIR}"/netboot
 	tar -C "${DESTDIR}" -chzf ${OUTDIR}/${output_filename} boot/
 }