openthread: Fix OpenThread independence from Zephyr network.

If CONFIG_OPENTHREAD_SYS_INIT is enabled, OpenThread initialisation
should also consider running OpenThread automatically if
CONFIG_OPENTHREAD_MANUAL_START is disabled.

Removed also dependency on the `net_bytes_from_str` functions from
the openthread.h file. Now, in the OpenThread module, there is an
additional `openthread_utils.c/.h` file that can implement useful
utilities for the OpenThread platform. Currently, it implements
the `bytes_from_str` function to use it instead of
`net_bytes_from_str`.

Signed-off-by: Arkadiusz Balys <arkadiusz.balys@nordicsemi.no>