Remove uses of the deprecated timeout_t typedef.

[jhb]

Summary

In some cases, remove casts by changing callout functions to accept
a void * argument and adding an assignment to a pointer of the desired
type. In other cases, expand the typedef.

Test Plan

• compile tested, booted in an amd64 VM

[kib]

I do not object, but what is wrong with the typedef itself ? It is in fact useful, since you have to spell the function pointer type explicitly more than once, esp. in casts.

[jhb]

The typedef itself is for the old API that I'm planning to remove. It's name doesn't make much sense for callout_*(). We could perhaps define a new callout_handler_t or some such in callout.h, but it's not clear to me that would really be better. The only casts here are 2 in uart_core.c. I don't mind renaming the typedef and keeping it for the uart casts (and using it in callout.h and a few other places in follow-on patches), but I don't think the timeout_t name or systm.h header are the right ones going forward.

[jhb]

Superseded by D22751 and D22752.