Implement config file
This MR introduces per-device configuration files in TOML format. These files allow configuring:
- the AT commands issued on modem startup, suspend, resume and reset
- which host GPIOs are used for modem interaction
- whether the modem needs to be discovered using libusb or not
- the modem detection timeout on resume
Config files are named after the host compatible
string, and can easily be overridden by the user/distro: they're installed into /usr/share/eg25-manager
by default, but are searched for in /etc/eg25-manager
first.
Fixes #6 (closed)