x86: Add console command to display CBMEM console buffer

This command is useful to allow to observe messages generated by
coreboot and u-boot until present. In particular it is handy when
u-boot is instrumented to fall through into console mode on startup
errors.

Signed-off-by: Vadim Bendebury <vbendeb@chromium.org>
Signed-off-by: Simon Glass <sjg@chromium.org>