[Design] PSU firmware update

[Lei YU]

> > ## Requirements
> >
> > To mitigate the risk of power loss, the PSU firmware code update shall meet
> > pre-conditions:
> > 1. The host is powered off;
> > 2. The redundant PSUs are all connected;
> > 3. The AC input and DC standby output shall be OK on all the PSUs;
>
> As part of the PSU code update, it will turn off it's control supply.
> If only 1 PSU has AC applied, and it turns off the control supply, then
> the BMC would lose power and get reset, which is not wanted.  That being
> said, some systems may have 4 PSU instead of 2.  So ALL of the redundant
> PSUs wouldn't be connected, but at least 1 other PSU should be connected
> and have AC applied, so that when 1 PSU is reset due to the download, at
> least 1 other PSU will hold up the control supply and be providing
> standby power to the BMC.  It should still be ok to download the
> remaining PSU even if they don't have AC applied.
>
> So #2 and #3 aren't always valid, including in systems with more than 2
> PSUs attached.  It's probably better to say that whenever downloading a
> PSU, that at least 1 other PSU is connected and has AC attached.

Yes, technically, as long as there is at least 1 other PSU has AC, it should
be safe to do PSU update.
But that involves a certain complexity, and it is safer to require all PSUs to
have AC.
So I chose a simpler way to require the above 2 & 3.

The requirements could be relaxed, let's see others' ideas probably?