cbmem_console: Improve 'cbmem -1' behavior for truncated pre-CBMEM logs

cbmem_console: Improve 'cbmem -1' behavior for truncated pre-CBMEM logs

The 'cbmem -1' flag that cuts off console output before the last boot
will ignore content from earlier stages if it was truncated due to lack
of pre-CBMEM console space. This patch makes the "log truncated" message
more specific and adds it as an additional cut-off marker to 'cbmem -1'
to counteract that problem.

Also raise the log level of the coreboot banner one step to BIOS_NOTICE
to make it more likely to be included in the output for 'cbmem -1' to
find. (I believe NOTICE is reasonable but I wouldn't want to go as far
as WARN which should be reserved for actual problems. Of course this is
not ideal, but then again, our whole log-level system really isn't... it
would be better if we could make it always print a banner to the CBMEM
console without affecting the UART at the same time, but that would
require a larger amount of work.)

Change-Id: I58288593dfa757e14f4a9da4ffa7e27b0b66feb9
Reported-by: https://ticket.coreboot.org/issues/117
Signed-off-by: Julius Werner <jwerner@chromium.org>
Reviewed-on: https://review.coreboot.org/19720
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Aaron Durbin <adurbin@chromium.org>