package/modem-manager: init script for systemV-like init systems

ModemManager get started by NetworkManager, in case of systemd init
system. In case of other systems it needs to be started by init script.

Debian [1] solved it by detection in code. For Buildroot it's IMHO
enough to install init script for systemV-like init systems.

[1] https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=770871



[Thomas:
 - slightly simplify the script by removing the MODEMMANAGER_BIN
   variable which was used at only one place, and use directly $?
   instead of an intermediate $ret variable.
 - split the too long line added in the .mk file.]

Signed-off-by: Petr Vorel <petr.vorel@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>