setup-disk: install syslinux when installing on mounted dir

ref #673

setup-disk.in

@@ -369,7 +369,7 @@ stop_all_raid() {
 init_progs() {
 	local raidpkg=
 	[ -n "$USE_RAID" ] && raidpkg="mdadm"
-	apk_add -q sfdisk e2fsprogs lvm2 $raidpkg syslinux $@
+	apk add -q sfdisk e2fsprogs lvm2 $raidpkg syslinux $@
 }
 
 show_disk_info() {
@@ -856,6 +856,7 @@ shift $(( $OPTIND - 1))
 
 if [ -d "$1" ]; then
 	# install to given mounted root
+	apk add -q syslinux
 	install_mounted_root "${1%/}"
 	exit $?
 fi