x86: Make chromebook_link the default board for coreboot

Change SYS_CONFIG_NAME and DEFAULT_DEVICE_TREE to chromebook_link
which is currently the only real board officially supported to run
U-Boot loaded by coreboot.

Note the symbolic link file chromebook_link.dts is deleted and
link.dts is renamed to chromebook_link.dts.

To avoid multiple definition of video_hw_init, the CONFIG_VIDEO_X86
define needs to be moved to arch/x86/cpu/ivybridge/Kconfig.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Simon Glass <sjg@chromium.org>