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

[Julius Werner]

> I'd also like the interface to be friendlier. What granularity is
> actually supported for the option? I doubt that it's single bytes?
>
> Maybe make it decimal kb? The default value depends on ROM_SIZE, right?

I would very much prefer it to stay byte sized. This makes the
Makefiles that use it easier to write and avoids confusion compared to
the byte sized CONFIG_ROM_SIZE. (IMHO building a unit like kilo or
mega into a variable or interface is always the less desirable choice
because it adds another opportunity to screw up and become confused
for the programmer. Sometimes it can be justified with the risk of
overflow of the respective data type, but for Kconfig that doesn't
count.)

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.