Protocol examples used to raise an error if the target was not set
and `idf.py build` command was used. This commit fix this error
and when IDF_TARGET is not set, ESP32 is selected as default target
This updates the minimal supported version of CMake to 3.16, which in turn enables us to use more CMake features and have a cleaner build system.
This is the version that provides most new features and also the one we use in our latest docker image for CI.
