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: private include header
add: macro encapsulation for assertion or error check
add: ESP_FAIL return code documentation in public headers
change: replaced all assertions by NVS_ASSERT_OR_RETURN macro
change: few internal function return values from void to esp_err_t
change: ESP_ERR_NVS_VALUE_TOO_LONG macro comment
* General tests like page loading from flash
* Rough test of fixed-size data types
* Rough test of blob read
* Added coverage target in cmake, also accessible
via `idf.py coverage`
* Fixed unsigned comparison in comp. enum table
* introducing temporary LINUX_TARGET define