net: openthread: Use Zephyr's mbedTLS instead of compiling own

So far OpenThread compiled it's own, internal copy of mbedTLS library.
This commit changes this behavior by using Zephyr's mbedTLS instance
appropriately configured for OpenThread needs.

Generic mbedTLS config file was used in this case, so that application
can still configure remaining parts of mbedTLS for it's own needs.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>