x86: Rename coreboot-x86 to coreboot

We only use coreboot as a target on x86 platforms, since on ARM platforms
U-Boot always runs as the primary boot loader. Rename the coreboot-x86
platform to reflect this.

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