Feature request: Ability to add custom defines

[rob-clarke]

The ability to add arbitrary #defines would be welcome. e.g. in the AP_TemperatureSensor library, it would be nice to be able to:

• set the number of sensors by defining AP_TEMPERATURE_SENSOR_MAX_INSTANCES
• disable TSYS03 default address checking by defining AP_TEMPERATURE_SENSOR_TSYS03_ENFORCE_KNOWN_VALID_I2C_ADDRESS=0

This is done with waf using --define. A text box / form to add #define names / values could be added to allow the equivalent from the custom build server.