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

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>