[test] Made the tests compile again with the current locm3 and changes to cdcacm.

Warning: the tests are not tested... :P
[Make] Enabled tests in the all target.
[Make] Enabled full path printing in the make errors.

If we have them hidden then tools like sublimetext are not able to find
the location of the files generating the errors.
[Make] Added prefix handling to the flash target.

- We need to be able to provide an arbitrary prefix to the DFU flash
  script. Otherwise we can't reuse the same Makefile.include in tests
  and the main src ridectory.
- Relocated the code generated by the multi cdcacm test behind the
  luftboot bootloader.
[test/transfer] Transfer example is now using the board file for portability.
[Make] Added multi_usb_cdcacm test to the test target.
[cdcacm] Converted the cdcacm module to use ring buffers.

Converted the cdcacm module to use ring buffers instead of callbacks for
the incoming and outgoing data transfers. We are using a fake process (a
function called periodically from the main loop) to process the outgoing
data and transfer it to the endpoint.

The test works but I can confirm that it is corrupting data. That is
already better than the crashing we were getting before. :)
[usbrf] Made the main code compile again.

(This definitely does not mean it works)