[poky] Processing extra field requested by bitbake -u in Cache

[Ke, Liping]

> >
> > Considering the following usage sequence, ui 1 -> non-ui (made changes) 2 ->
> ui (made changes)  3 -> non-ui (made changes) 4
> >
> > If we use option 1, no cache invalidation needed.
> 
> If there are changes you'll still need to invalidate the cache.
> I guess you're trying  to show something like the following?
> 
> == Option 1 (addition cache file for extra fields)
> launch gui
> <make changes>
> launch gui (cache rebuild)
> use cli
> <make changes>
> use cli (cache rebuild)
> launch gui
> use cli
> launch gui
> <make changes>
> launch gui (cache rebuild)
> use cli
> 
> => 3 cache rebuilds
> 
> == Option 2 (per ui cache files)
> launch gui
> <make changes>
> launch gui (cache rebuild)
> use cli
> <make changes>
> use cli (cache rebuild)
> launch gui (cache rebuild)
> use cli
> launch gui
> <make changes>
> launch gui (cache rebuild)
> use cli (cache rebuild)
> 
> => 5 cache rebuilds
> 
> However I can't imagine people switching UI's this frequently.
> 
Hi, Josh
Yes, I asked whether we would like to use the image creator, some of them said yes, they will.
We're all waiting on functionality: select a image type, and add or move a recipe and then make rootfs...
If this functionality is ok, we guess image creator would be very useful.
I guess it's really very hard to judge users' usage scenario?

Yes. Reparsing will take extra 30s. I found it's a hard decision since we're always trying to improve the
Performance and reduce the disk size. 

criping
> >
> > But if we use option 2, we need invalidate cache each time. I collect some
> local feedback here, they said it's unbearable.
> 
> What's unbearable? The cache rebuilding time?