USART: Add usart_info command

This optional console command is enabled with CONFIG_USART_INFO_COMMAND.
It will display and clear dropped character and overrun counts for all
configured USARTs.

Signed-off-by: Anton Staaf <robotboy@chromium.org>

BRANCH=None
BUG=None
TEST=make buildall -j

Change-Id: Icf6061aaab2cda71e9d317455c897828b9daf844
Reviewed-on: https://chromium-review.googlesource.com/292770
Tested-by: Anton Staaf <robotboy@chromium.org>
Reviewed-by: Randall Spangler <rspangler@chromium.org>
Commit-Queue: Anton Staaf <robotboy@chromium.org>
Trybot-Ready: Anton Staaf <robotboy@chromium.org>
12 files changed