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.
* add toolchain file
* add linux to preview targets
* add stub for dfu number in cmake
* excluded unity runner per default
* Added esp_attr.h and esp_partition.h linux stubs
* component.cmake check list for emptyness
* added switch for linux in unity cmake file
* Added Linux host example app