nissa/nereid: shrink hostcmd console buffer
nissa/nereid: shrink hostcmd console buffer

We need more memory for the USB mux task, and this buffer is a convenient
place to get it from, at the cost of getting less history from
logs gathered by the host processor.

This change also removes the board-specific serial buffer size option,
since it's also set in prj.conf.

BUG=b:223044986,b:223749568
TEST=nereid build succeeds with USB mux task enabled
BRANCH=none

Signed-off-by: Peter Marheine <ef0b2c492edde94778d7465bfbaeed52c5be6858@chromium.org>
Change-Id: I68db610756fd80135529cd7eda582ad1f697d042
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/3524064
Reviewed-by: Andrew McRae <44ad1424cfc8411c96d9181798ca41ae55709dfb@google.com>
Commit-Queue: Andrew McRae <44ad1424cfc8411c96d9181798ca41ae55709dfb@google.com>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec