Adding BuildDate field to entity-manager output

[Patrick Venture]

So, with entity-manager you can specify and interface, some properties
and how the values should be presented.  For the BuildDate field
however, the output from FruDevice for that interface doesn't match
the Asset interface's date property format.

I merged patch into phosphor-host-ipmid that makes it support parsing
multiple formats, however, that dbus interface defines the format "by
description."

I was curious if it would be worth trying to reinterpret the value
when publishing it to that dbus interface?  Converting from "%a %b %e
%H:%M:%S %Y" to "%Y-%m-%d - %H:%M:%S"

Honestly, I'd be perfectly fine just adding it as-is to the Asset
interface description since phosphor-host-ipmid knows how to parse it
(pending: https://gerrit.openbmc-project.xyz/24384).

Thoughts?

Patrick