hostcmd: Flush UART before doing cold reboot
hostcmd: Flush UART before doing cold reboot

Flush our UART buffer to ensure that we don't miss prints when we reboot
the EC.

BUG=chrome-os-partner:55539
BRANCH=None
TEST=Manual on kevin, issue cold reboot host command, verify that
"Executing host reboot command" is seen on console.

Change-Id: I96d5687b413ba4f603e3e7845b5cbba1c2d65efa
Signed-off-by: Shawn Nematbakhsh <5259333ce42e3ace6a73988cec7115ed9d3a661a@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/362681
Commit-Ready: Shawn N <5259333ce42e3ace6a73988cec7115ed9d3a661a@chromium.org>
Tested-by: Shawn N <5259333ce42e3ace6a73988cec7115ed9d3a661a@chromium.org>
Reviewed-by: Aseda Aboagye <12c9b286316a940fd31c24070da5ab64cdd4d0e7@chromium.org>

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