[coreboot] Fixing payload address

[Trammell Hudson]

I'm working with a fairly large Linux payload in my coreboot image
and one of my targets (the x230) has two separate ROM chips.  I'd like
to have the top 4 MB SPI flash reserved for coreboot (bootblock,
romstage, ramstage, mrc, etc) and the bottom 8 MB chip just for
Linux.

Most of my changes now are in the payload, not in the coreboot
components, so ideally I would not need to touch the top 4 MB
chip at all.  I can't figure out a way to ask CBFS to create
two sections like this for me.  Is it possible?

As a secondary improvement, I'd like to have an even more minimal
Linux payload in the top 4 MB that has spiflash tools to re-write
the bottom 8 MB.  Is there an easy way to select which payload
will be executed?