Windows CI: Appveyor based SITL build using Cygwin Toolchain

[MaEtUgR]

As requested by @dagar and proven to be necessary multiple times: I want to add CI build support on Windows such that it's easily visible to developers when the build breaks in the Cygwin toolchain described here: https://dev.px4.io/en/setup/dev_env_windows_cygwin.html

References: #9981 , #9920

[MaEtUgR]

From #9981

Update: I worked on the CI using Appveyor, it only doesn't build master yet because something with numpy on the latest cygwin is buggy and it doesn't import correctly.
And I still have to figure out how to checkout the correct branch. But it looks like I'm almost there thanks to the script based toolchain setup repo I did before which only required some small adaptions.

[dagar]

Appveyor is likely the best solution if possible. It also serves as a nice test for the installer.

If that's not workable for whatever reason, then worst case we could add a Windows machine to Jenkins. Someone would need to manually setup (and maintain) the dev environment though.

[MaEtUgR]

Right now I did it with scripts always installing everything fresh from the internet. I can do the installer later as well.

Someone would need to manually setup (and maintain) the dev environment though.

I could easily do that.

[MaEtUgR]

@dagar Finally I have the first successful appveyor build of sitl and px4fmu-v4 🎉 :
https://ci.appveyor.com/project/MaEtUgR/firmware-b5mk6/build/23
Had to find multiple good hiding script details to make it work. Now I create a pr and explain more there.