modem-manager: support a defaults file in the init script

Sometimes it is useful to pass some parameters to ModemManager when it
starts (e.g. --log-level). Allow the user add a file with such flags in
a MODEMMANAGER_ARGS variable. This is simpler than overriding the whole
startup script (e.g. by means of a rootfs overlay).

Signed-off-by: Carlos Santos <casantos@datacom.ind.br>
Acked-by: Petr Vorel <petr.vorel@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/modem-manager/S44modem-manager

@@ -3,13 +3,16 @@
 # Starts ModemManager
 #
 
+# Allow a few customizations from a config file
+test -r /etc/default/ModemManager && . /etc/default/ModemManager
+
 PIDFILE=/var/run/ModemManager.pid
 
 start() {
 	printf "Starting ModemManager: "
 	umask 077
 	start-stop-daemon -S -q -b -m -p $PIDFILE \
-		--exec /usr/sbin/ModemManager
+		--exec /usr/sbin/ModemManager -- $MODEMMANAGER_ARGS
 	[ $? = 0 ] && echo "OK" || echo "FAIL"
 }
 stop() {