[coreboot] Kconfig: Move `CBFS_SIZE` out of main section

[Julius Werner]

> It's trivial to add another variable between the user interface and
> Makefile rules to turn the friendly value into what's convenient for
> the code.

Yeah. But then again, we already agreed that this is a very rare-use
expert option. I don't think it's worth adding yet more lines to the
Kconfig file just to make it pretty to choose.

>> The actually supported granularity is anything that CBFS/cbfstool
>> supports as an image size, which I think is in theory arbitrary down
>> to individual bytes.
>
> What are the common CBFS_SIZE values?

We're using 1MB for most (all?) of our boards, but that's really
completely arbitrary. If you want to use this at all, you probably
know exactly what kind of size you want it to be as well. I really
don't see the point in expending effort to restrict this to some
arbitrary bin size.

(I'd also like to add that we're already developing a half-finished
feature in our tree right now that will reshuffle the whole image
layout once again, making this option obsolete... so we might not want
to spend too much effort polishing it.)