elm: Increase UART TX buffer size to 8kb
elm: Increase UART TX buffer size to 8kb

We have more than enough memory for that, and it makes it possible
to poll the logs from AP much more unfrequently.

BRANCH=oak
BUG=chromium:527904
TEST=make buildall -j
TEST=Boot elm, cat /sys/kernel/debug/cros_ec/console_log does not
     miss any data.

Change-Id: I8cce88e39d00a94397b6fc852a371b4595870b24
Reviewed-on: https://chromium-review.googlesource.com/358181
Commit-Ready: Nicolas Boichat <f3daba1cdc403799935674d3cd4b0e5bd1804232@chromium.org>
Tested-by: Nicolas Boichat <f3daba1cdc403799935674d3cd4b0e5bd1804232@chromium.org>
Reviewed-by: Gwendal Grignou <190c5a5edfcf62e0246559bf31fa7548674de3ad@chromium.org>

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