Draft: Implement initial functionality

This implements initial functionality to use QMI modem(s) as wireless WAN.

Only one modem at the time acting as WAN is currently supported. The expected setup is just one modem - i.e. Turris router with LTE kit - but code can handle multiple attached modems.

In that scenario all detected modems are offered as a choice, but there can be only one selected and designated as WAN.

Following API functions are currently implemented:

• get_settings
• update_settings

This MR depends on functionality implemented in: foris-controller!125

foris_controller_backends/wwan/__init__.py

@@ -192,6 +192,14 @@ class WWanCmd(BaseCmdLine):
         just whether SIM card is 'working' or not.
         """
         try:
+            # There are two utilities to fetch information from QMI modem - 'qmicli' and 'uqmi'.
+            # 'qmicli' is more recent and it looks like it is actively
+            # maintained, but at the moment it can produce only text output.
+            # 'uqmi' is older and it could get stuck while trying to fetch SIM
+            # state informations on certain modems. But it produces json output.
+            #
+            # Therefore I decided to go with 'uqmi' and set timeout for its
+            # process, in case it gets stuck.
             stdout, _ = WWanCmd._run_command_and_check_retval(
                 ["/sbin/uqmi", "-d", ctl_device, "--uim-get-sim-state"],
                 expected_retval=0, timeout=20