segger: Resolve circular include dependencies

Zephyr kernel is dependent on trace.
Trace is dependent on segger rtt.
Segger rtt MUST NOT be dependent on zephyr kernel.

Move lock functions from header into c file to avoid circular
dependency. This fix needs an update of the segger repository.

Fixes #43887.

Signed-off-by: Christoph Coenen <ccoenen@baumer.com>