x86: Update cbmem driver

This driver is not actually built since a Kconfig was never created for
it.

Add a Kconfig (which is already implied by COREBOOT) and update the
implementation to avoid using unnecessary memory. Drop the #ifdef at the
top since we can rely on Kconfig to get that right.

To enable it (in addition to serial and video), use:

   setenv stdout serial,vidconsole,cbmem

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
[Modified the comment about overflow a little bit]
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>