x86: coreboot: Use vendor in the Kconfig

Use VENDOR_COREBOOT instead of TARGET_COREBOOT so we can have multiple
coreboot boards, sharing options. Only SYS_CONFIG_NAME needs to be
defined TARGET_COREBOOT.

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

arch/x86/cpu/coreboot/Kconfig

@@ -1,4 +1,4 @@
-if TARGET_COREBOOT
+if VENDOR_COREBOOT
 
 config SYS_COREBOOT
 	bool