x86: coreboot: Use vendor in the Kconfig

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>