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

[Julius Werner]

> commit f780c40f (CBFS: Correct ROM_SIZE for ARM boards, use CBFS_SIZE
> for cbfstool) [1] adds a description to the Kconfig variable
> `CBFS_SIZE`, which makes it, I think, visible in the Kconfig menu (`make
> menuconfig`).

Sorry about that. The menuconfig for the Chromium tree I originally
wrote this for was already so cluttered with random stuff on the main
page that I didn't think anyone cared. ;)

> What section should
>
>         (0x400000) Size of CBFS filesystem in ROM
>
> be moved to?

The option is closely related to ROM_SIZE, so maybe put it under
'Mainboard -->' with that? General setup could also make sense, I
guess.

> Furthermore, should it even be visible or be hidden as an expert option?

Hiding it as an expert option probably makes sense. It is used to
restrict the amount of space coreboot (CBFS) takes up on the ROM in
case you want to put other stuff on there as well, which is probably a
pretty limited use case.