[U-Boot] [PATCH 3/5] x86: Add CBMEM console driver for coreboot

[Simon Glass]

From: Vadim Bendebury <vbendeb at chromium.org>

This patch builds upon the recently introduced CBMEM console
feature of coreboot.

CBMEM console uses a memry area allocated by coreboot to store
the console output. The memory area has a certain structure,
which allows to determine where the buffer is, the buffer size
and the location of the pointer in the buffer. This allows
different phases of the firmware (rom based coreboot, ram based
coreboot, u-boot after relocation with this change) to keep
adding text to the same buffer.

Note that this patch introduces a new console driver and adds the
driver to the list of drivers to be used for console output, i.e.
it engages only after u-boot relocates. Usiong CBMEM console for
capturing the pre-relocation console output will be done under a
separate change.