HAL: Allow SERIAL_PPM to be overwritten by defines (buildserver)

I removed the default define for  ```SERIAL_PPM``` like you did with ```RC_MAPPING```.

But neverless it seams the Define from the buildserver for at least ```SERIAL_PPM``` does not work at all.
Generated hex file is identical with the hex generated after the fix

```b353ce1f34bdd417cd51c56a05434f44_5196a9a05ddfa0a726ff2977cae555e5a60ad505.hex``` vs ```b353ce1f34bdd417cd51c56a05434f44_4e5d5efb5dbc52101be238fb399e3551ea36e392.hex```

You mentioned in another issue that you did not add the define for this input settings to the APM_Config.h because its out of scope of the library. it is possible that this is the same problem? How could we fix that, at least for the Build server.