modem: Add minimal GPRS support

Up to this point we can test signalling plane: attaching to GPRS network and activating a
context.

Data plane (sending IP packets through the network) is still not
implemented as it requires setting up the network interface provided by
ofono as well as routing, and most probably move osmo-ggsn to its own
network namespace.

Change-Id: I605ba1bb1103a045a9b5d0e7215c05dfc1fe575f
1 file changed