openthread: kconfig: rework OpenThread security configurations

With the updates to mbedTLS Kconfig it is now possible to update the
OpenThread security configurations by disabling the mbedTLS prompt and
avoid stuck symbol selection.

As part of this, the OpenThread security selection has been reworked
into a choice which ensures only a single security selection can be
chosen.

And the OPENTHREAD_MBEDTLS itself has been made promptless to ensure
other parts of the build system can select a specific OpenThread
security implementation and disable user selection, if the module or
sample require such behavior.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>
Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>