all: rework routers ntp waiting mechanism

Currently, on routers that require NTP waiting, ctrld makes the cleanup
process, and restart dnsmasq for restoring default DNS config, so ntpd
can query the NTP servers. It did work, but the code will depends on
router platforms.

Instead, we can spawn a plain DNS listener before PreRun on routers,
this listener will serve NTP dns queries and once ntp is configured, the
listener is terminated and ctrld will start serving using its configured
upstreams.

While at it, also fix the userHomeDir function on freshtomato, which
must return the binary directory for routers that requires JFFS.

resolver.go

@@ -201,3 +201,17 @@ func lookupIP(domain string, timeout int, withBootstrapDNS bool) (ips []string)
 	}
 	return ips
 }
+
+// NewBootstrapResolver returns an OS resolver, which use following nameservers:
+//
+//   - ControlD bootstrap DNS server.
+//   - Gateway IP address (depends on OS).
+//   - Input servers.
+func NewBootstrapResolver(servers ...string) Resolver {
+	resolver := &osResolver{nameservers: nameservers()}
+	resolver.nameservers = append([]string{net.JoinHostPort(bootstrapDNS, "53")}, resolver.nameservers...)
+	for _, ns := range servers {
+		resolver.nameservers = append([]string{net.JoinHostPort(ns, "53")}, resolver.nameservers...)
+	}
+	return resolver
+}