This commit adds an initial set of VTY commands to manage APN
configuration and set up, which is used by the modem app.
The sample modem.cfg file is updated to showcase how to configure APNs.
The app doesn't do anything with them yet. A follow up patch will add
code to create tun devices for each configured APN.
Change-Id: I7b4eaa0de428b418bb1d89bd544694e89beb3e6e
This allows apps to allocate the objects as they please: simple apps can
statically allocate it at startup. Others may want to allocate them
through VTY.
Some apps may also want to dynamically start and stop layer2.
Change-Id: I32f99df76a5513eff9df5489d28d60aedf96dec3
This way we can extend its API and contents more easily,
and keep most of it together in one place.
Change-Id: Icb4891cc1e4a0ecb5f09cb8a84b0ebe1b91a46b8