Improving logging

[shakthi-prashanth-m]

Right now, we're logging DroneCore logs on console. Generally its a practice to dump all the debug logs to a file. We could save them onto say, build/default/logs/dronecore.log.

Then, a user may simply run on a terminal: tail -f build/default/logs/dronecore.log

So, we can add more debug/info logs in the code (as it won't collide with application console logs) which are really helpful to analyze issues. For ex: say, arming/takeoff fails. We can easy look into logs to see whether all the necessary things went well or not.

PS: As discussed in Slack, we're planning to add this in future (not right now).

[julianoes]

Additionally, we should add a method to specify the log level.