sysntpd: restore support for peer-less (standalone) mode

ntpd from Busybox supports peer-less (standalone) mode when it's started
with option -l and without any peer provided with option -p. In this
mode ntpd uses local time as reference and acts as stratum 1 server.

This mode can be used in isolated networks, where Internet access and/or
other NTP server/s are not available, but the device has some other way
of getting correct time, like e.g. GPS (ugps supports setting local time
by default).

Support for this mode was incorrectly disabled/removed in:
1527f96ca6

Signed-off-by: Piotr Dymacz <pepe2k@gmail.com>
Acked-by: Jo-Philipp Wich <jo@mein.io>

package/utils/busybox/files/sysntpd

@@ -45,7 +45,7 @@ start_service() {
 
 	[ $use_dhcp = 1 ] && get_dhcp_ntp_servers "$dhcp_interface"
 
-	[ -z "$server" ] && return
+	[ -z "$server" -a "$enable_server" = "0" ] && return
 
 	procd_open_instance
 	procd_set_param command "$PROG" -n -N