Add a hook to run code from user_init

[igrr]

As seen in #2111, would be useful to disable WiFi early, and maybe for some other cases as well.

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[devyte]

@igrr I suggest pushing this one back to 2.5.0 (unless it's trivial to implement?)

[igrr]

Sure. At this point i am not planning on adding more features to 2.4.0, only bug fixes are considered. I guess we can mass-reassign all feature requests from 2.4.0 to 2.5.0.

[d-a-v]

Closed by #5395