[qmp-system] Activate wifihealth script by default

And run it via cron every 5 minutes.

Closes #470.
Signed-off-by: Roger Pueyo Centelles <roger.pueyo@guifi.net>

packages/qmp-system/files/etc/uci-defaults/qmp_wifi_health_cron_patch

+#!/bin/sh
+
+if ! cat /etc/crontabs/root | grep wifihealth; then
+	echo "*/5 * * * * /usr/bin/wifihealth.sh >> /tmp/log/wifihealth.log" >> /etc/crontabs/root
+	/etc/init.d/cron enable
+	/etc/init.d/cron start
+fi