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>
diff --git a/configs/chromebox_panther_defconfig b/configs/chromebox_panther_defconfig
index 0613cd6..c4db232 100644
--- a/configs/chromebox_panther_defconfig
+++ b/configs/chromebox_panther_defconfig
@@ -1,5 +1,6 @@
 CONFIG_SYS_EXTRA_OPTIONS="SYS_TEXT_BASE=0xfff00000"
 CONFIG_X86=y
+CONFIG_VENDOR_GOOGLE=y
 CONFIG_TARGET_CHROMEBOX_PANTHER=y
 CONFIG_OF_CONTROL=y
 CONFIG_OF_SEPARATE=y