weak hook preinit() #2111 #2133 #2136

[d-a-v]

Called in init_done() callback after global constructors initialized.
Or should it be called in user_init() (before global constructors) ?
#2111 #2133 #2136

[earlephilhower]

Can we get a simple example to disable the WiFi stuff like in the issues? That's almost the only reason to have this.

Also need some kind of document about this weak call:
. No C++ allowed, no global objects, no interrupts or exceptions
. This is run in SYS context and can't take too long or yield/delay

[earlephilhower]

Assuming the IP changes are intended, LGTM.