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

[Myles Watson]

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?

This allows v2 to load "legacybios" without having to have a separate loader.

Signed-off-by: Myles Watson

Thanks,
Myles