Add custom defines support

[rob-clarke]

Adds a text box to the bottom of the new build page to input custom #defines. Input is in DEFINE=VALUE, DEFINE=VALUE format. These are parsed and injected into the extra_hwdef.dat file when setting up the build.

Any custom defines are also output in the build log:

Tested locally in Docker and successfully built a version with custom #defines injected. Leaving the box blank results in the current behaviour.

Addresses #217

[shiv-tyagi]

Hey @rob-clarke, thanks for submitting the PR.

I honestly don't like the way the text box is placed in the form. A text area element would better suite such use case. It should also be preferably hidden.

I am also not sure if we should allow a user to add anything to the hwdef. Introducing a new feature type supporting non boolean values would make more sense. So that we present the feature name to user and user just types the value for it.

But I would like others to put their views in.

[rob-clarke]

I did consider hiding the text field behind an "advanced" button or similar, probably a good idea.

The use case I have would be covered by adding an integer type, but there may be others that would benefit from a free-form input e.g. if the value should depend on another #define value (though not sure how that would work with hwdef).

I guess there could be work on adding all "configurable" #defines to build_options.py or similar to provide the metadata to enable other feature types, but that would then only be compatible with future releases.