Introducing Flapper platform

[matejkarasek]

This PR adds a "flapper" platform, together with default settings and gain values.

• To enable servomotors, which need a non-zero neutral value, additions were made to Motors.c and supervisor.c
• depends on PRs Improved support extRx (CPPM) #1121 and Migrating default PID gains and filter settings to platform defaults and Kconfig #1122, which are also included in this PR

[knmcguire]

Could you resolve the build issue that the CI caught? You can check out the log in details

[krichardsson]

Does this PR depend on 1122? Does it have to be merged first?

[matejkarasek]

Does this PR depend on 1122? Does it have to be merged first?

I have included the original commits from #1122 (and #1121) also in this PR, so it is already merged.

I am working on fixing the remaining issues.
Next to the feed-forward gains mentioned in the discussion of #1122 but left out eventually, I have see redefinition issues with when multiple power distributions are used, will see, what can be done about that

[krichardsson]

@matejkarasek maybe you should consider creating a new PR for adding the flapper?
I have merged your other two PRs and I think it would be cleaner to make a new branch from master for this PR. Currently this PR contains all the changes from the other two and it is hard to understand what is flapper related.

I have also merged a PR that updates the build scripts a bit and all you have to do build flapper in CI is to add i to the build_targets.json file

[matejkarasek]

Yes, will do that soon, thanks!