modemmanager: add possibilty for setting initial EPS bearer

modemmanager: add possibilty for setting initial EPS bearer

If no GSM but only 4G is available and a special APN must be used, it
is necessary to set an inital EPS bearer beforehand. If this is not set,
then modem cannot log in and register in the mobile network.

The new option 'init_epsbearer' could be set to the following options.
* none: No init EPS bearer is used and the old one is deleted (default)
* default: Use init EPS bearer with the following config options
  'iptype', 'allowedauth', 'password', 'user' and 'apn' as for the
  connection bearer.
* custom: Other parameters are used that do not match those of the
  default connection bearer. These have an 'init_' prefix and are named
  in the same way as the default connection bearer config options.

Signed-off-by: Florian Eckert <fe@dev.tdt.de>