modules: mbedtls: set mbedTLS debug threshold during module initialization

mbedTLS library threshold initialization was done in native TLS socket
implementation (which tends to use mbedTLS now) and inside mbedTLS
benchmark test. Move that to mbedTLS module initialization, as this is a
global setting.

Update description of CONFIG_MBEDTLS_DEBUG_LEVEL to clarify when
mbedtls_debug_set_threshold() is called.

Signed-off-by: Marcin Niestroj <m.niestroj@emb.dev>