modemmanager: delay the launch of MM for XBee Cellular

The XBee Cellular needs more time to settle down before MM is able to
detect it on boot. As this is connected to one of the 'ttymxc' serial
ports, use that port to conditionally delay the launch of MM.

Signed-off-by: Javier Viguera <javier.viguera@digi.com>

meta-digi-dey/recipes-connectivity/modemmanager/modemmanager/cellularifupdown

@@ -7,6 +7,13 @@ POLLING_WAIT=5
 [ -z "${IF_APN}" ] && exit 0
 
 background_connect() {
+	# The XBee Cellular needs more time to settle down before MM is able
+	# to detect it. As the XBee is connected to a 'ttymxc' serial port,
+	# we check those ports to conditionally delay the launch of MM.
+	if echo "${IF_PORT}" | grep -qs ttymxc; then
+		sleep 25
+	fi
+
         # Ensure ModemManager is running
         killall -0 ModemManager 2>/dev/null || ModemManager --log-file=/var/log/modemmanager.log &