x86: Kconfig: Divide the target selection to vendor/model

Let arch/x86/Kconfig prompt board vendor first, then select
the board model under that vendor. This way arch/x86/Kconfig
only needs concern board vendor and leave the supported target
list to board/<vendor>/Kconfig.

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