[coreboot] v3 defconfig

[Myles Watson]

This patch adds support for make defconfig in v3.  Those that port v3
to a board should add a defconfig in mainboard/vendor/board/defconfig.
 I think that the defconfig should:

1. Use the ROM size that comes with the board
2. Enable compression
3. Not include a payload

This will make it easy for buildrom or anyone who wants to build it
manually to use lar to add their payloads. It also allows buildrom to
keep the configs in the coreboot tree.

The patch also adds mainboard/emulation/qemu-x86/defconfig

Should we change v2 to use the same naming?  It would be nice not to
have one be qemu-i386 and one qemu-x86.

Myles

Signed-off-by: Myles Watson