x86: coreboot: Rearrange arch_cpu_init()

Init errors in SPL are currently ignored by this function.

Change the code to init the CPU, reporting an error if something is wrong.
After that, look for the coreboot table.

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