libwebsockets

esp32: shift to support latest esp-idf

Esp-idf has an improved but still kind of abused cmake-
based build system now.

If we see ESP_PLATFORM coming as a cmake var, we can know we
are being built from inside the esp-idf config system.

Leave the existing esp32 arrangements alone but triggered off
ESP_PLATFORM, adapt to use the cross toolchain file and
various quirks automatically.

In this way you can build lws a part of your project in a
much cleaner way.

Prepare a minimal esp32 test app for use in Sai

Adapt .sai.json to build for esp32