openthread: Rework L2 configuration

- Add possibility to choose implementation of OpenThread L2 and
set it to Zephyr's by default
- Remove unused `OPENTHREAD_SECURITY_INTERNAL` Kconfig
- Add missing choice names for logging and coprocessor

Signed-off-by: Maciej Baczmanski <maciej.baczmanski@nordicsemi.no>
2 files changed