firmware: google: memconsole: Adapt to new coreboot ring buffer format

The upstream coreboot implementation of memconsole was enhanced from a
single-boot console to a persistent ring buffer
(https://review.coreboot.org/#/c/18301

). This patch changes the kernel
memconsole driver to be able to read the new format in all cases.

Signed-off-by: Julius Werner <jwerner@chromium.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>