This commit deprecates the "freertos/xtensa_api.h" and "xtensa/xtensa_api.h"
include paths. Users should use "xtensa_api.h" instead.
- Replace legacy include paths
- Removed some unnecessary includes of "xtensa_api.h"
- Replaced some calls with "esp_cpu_..." equivalents
- Add warning to compatibility header
1: add the sequence diagram for the bluetooth example projects,bt_l2cap_server and bt_l2cap_client, which implenment the L2CAP connection.
2: add the discription for the sequence diagram.
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.
