[coreboot] Coreboot-v2 qemu target rename

[Myles Watson]

Coreboot-v3 calls the qemu board qemu-x86, but Coreboot-v2 calls it
qemu-i386.  I think that qemu-x86 should be the preferred name.

Does anyone have any reservations about the switch to qemu-x86 in
Coreboot-v2?

Thanks,
Myles