remove default support for esp32

The esp32 core is too complex to maintain for the benefit of having
it. It

• uses xtensa-esp32-elf-gcc, which has a bunch of non-static binaries
• requires Python with specific libraries installed
• is a big download

It should still be possible to use this core provided you set up your
environment correctly and use the correct customization options.

support Yaml config file `.arduino-ci.yaml`

Now arduino-ci looks for the .arduino-ci.yaml config file in the top-level directory of your library. This is now the recommended way to configure arduino-ci.

Initial release

Merge branch 'dev/ryantm/in_ci' into 'master'

add __ARDUINO_CI define to compile args

See merge request dev/arduino-ci!5