During production it is often desirable for devices to perform
some kind of basic self-test to isolate defects.
For this it is necessary for the initialization not to hang if a
component is faulty / not connected.
This moves an already exising self-test that was previously enabled
as a debug option to an independent compile-time configurable.
It is necessary to call this in _init() before mrf24j40_hardware_reset()
as the reset function uses xtimer_usleep() which will cause another
thread to get scheduled.
If this thread (e.g. rpl or ipv6) then tries to access the netdev, RIOT
will crash.
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