Allow the modem to enter soft sleep when
we don't talk to the modem using AT commands.
This was already the case in suspend, but
not during runtime. By only waking the modem
from soft sleep when we need to send
an AT command, we can save some power.
This commit prepares the use of device-specific configuration files.
These files should be named after the device-tree `compatible` string
with the `toml` extension.
`eg25-manager` will search for config files in
`<prefix>/etc/eg25-manager` first (or `/etc/eg25-manager` if prefix is
`/usr`), then in `<prefix>/share/eg25-manager`.
When powering off the modem, we must assert the RESET line only when the
modem has shut down (otherwise it'll cause a hard reset and won't allow
the modem to shut down properly).
This commit also polls the STATUS pin during poweroff and quits
immediately once this pin goes high.
