USB init before using usb_serial_acm gadget?

[Sergey Nazaryev]

> So I think the right place to call usb_gadget_initialize is probably
> before registering the acm gadget function into acm_stdio_start(). Can
> you try this? and submit a follow_up fix patch if working?

It's required for `usb_gadget_initialize` to provide a "USB
controller index" as an argument. It's pretty obvious how to do it in
"interactive world" (e.g. to ask user for this number via CLI, like
it was in commands mentioned in my previous message) but in our case
we need to provide this number in non-interactive manner.

There is not an elegant solution with hardcoding '0' as a controller
index like it was done in `ether` gadget [1], but it seems that it
was a temporary fix ("This is a preparatory work for DM support for
UDC drivers") that occasionaly became a permament solution.

So, that's why I suggested to add a new subcommand to `usb` for OTG
initialization: in my opinion, it's better to require an explicit
call of something like `usb otgstart 0` than hardcoding a '0' into
the code. One more option is to add a new environment variable to
explictly specify a controller index, and read this variable from all
places when it requires.

P.S. I already put `usb_gadget_intiialize(0)` to my board code, and
ACM gadget started working; so the remaining problem is only to
decide where is the best place for this call.

[1]: https://source.denx.de/u-boot/u-boot/-/blob/master/drivers/usb/gadget/ether.c#L2355