mbedtls: Link FreeRTOS threading implementation at integration

Linking the `mbedtls-threading-freertos` CMake library to
`mbedtls-config` at the FRI component integration level
saves applications from having to include it thus
simplifyig the usage of the MbedTLS component.

Signed-off-by: Hugues Kamba-Mpiana <hugues.kambampiana@arm.com>