Feature request: Ability to add custom defines

The ability to add arbitrary `#define`s 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.

Start by tracing the custom build server's form or request handling and how it invokes waf with --define. The change is complete when users can enter arbitrary define names and values and those definitions reach the custom build equivalently to waf --define; the issue names no specific files or tests.

Written by the indexing model from the issue text.