[coreboot] Allow payloads to write over Coreboot tables, add a warning

[Stefan Reinauer]

Myles Watson wrote:
> This is a simple patch which allows payloads to be placed in memory in
> the range of 0xf0000-0x100000, where the Coreboot tables live in v2.
> As long as the payload doesn't need the tables, it seems harmless, so
> why not just print a warning?

It will definitely break your interrupt routing ... While I agree this
is OK for testing, we have to find a better solution for this.