mb/emulation: Select QEMU-i440fx by default

It's a better default than QEMU-armv7, which is currently the default
board when coreboot is configured for the first time, because most
coreboot development targets x86.

With this patch, the minimal steps to coreboot+SeaBIOS booting in QEMU
become:

    git clone https://review.coreboot.org/coreboot.git && cd coreboot
    make crossgcc-x86
    make olddefconfig && make
    qemu-system-x86_64 -bios build/coreboot.rom

Change-Id: Ie44a5d95547a55df93f29082c3b5a86fb83aa1e7
Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Reviewed-on: https://review.coreboot.org/16987
Tested-by: build bot (Jenkins)
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>

src/mainboard/emulation/Kconfig

@@ -2,6 +2,7 @@ if VENDOR_EMULATION
 
 choice
 	prompt "Mainboard model"
+	default BOARD_EMULATION_QEMU_X86_I440FX
 
 source "src/mainboard/emulation/*/Kconfig.name"