Enable VFS to be set via config

[tedwardd]

I've begun looking at how we can accept the VFS option via the meshtasticd config file in meshtastic/firmware. It is my understanding that -d/--fsdir are handled by PortduinoFS. Upon inspection, I see where we are calling PortduinoVFS->mountpoint() to find the value for the VFS but there does not appear to be any way to determine if this value was set via the runtime flag or is the default value without baking in assumptions about the default which seems fragile. I am admittedly new to C++ and may be missing something here but upon inspection of the framework-portduino codebase I'm able to find where we are evaluating the runtime arguments at setting _mountpoint but I see now indication that we're passing along any information about how we derived that value (whether it was passed at runtime or is the default value.

Assuming my understanding above is correct, I'd like to propose that we make available a value in PortduinoVFS to check whether or not we're using the default mountpoint or not. Alternatively, if it is acceptable, I would like to make framework-portduino aware of the -c/--config option and enable us to use a config value if one is provided. The order of preference should be Runtime flag -> config option -> default.