switch to the tracing subsystem of mbed #12

CMakeLists: Add option to enable tracing (on by default)

main: Only enable tracing if it's enabled in Mbed ...

GCC will whine about unused variables if we try to set up tracing and
Mbed implements it as a no-op. Wrap this in an a compile-time check.

Make use of the mbed tracing subsystem instead of basic printing ...

This enables use of the mbed tracing subsystem for the entire source
code of the project. All the uses of printfs are now appropriately
tagged and assigned a trace level, as in the following example:

[INFO][FS]: Unmounting the filesystem...
[INFO][FS]: OK
[INFO][USBM]: Switching to the usb mass storage mode...
[INFO][USBM]: Disconnecting the usb mass storage device...
[INFO][USBM]: Switched usb mass storage mode off.
[INFO][FS]: Mounting the filesystem...
[INFO][FS]: OK

This was also done in a way that the tracing is thread safe, using a
mutex to lock and unlock as needed.

Note that this cannot be turned on or off yet during build. It is
currently always enabled.