mtools: upgraded package to upstream

Upgraded sys-fs/mtools to version 4.0.23 for all.
This includes a runtime locking fix.
Upstream has integrated our patches, and fix a timing error in the lock
logic.  Upgrade to get them all.

This is needed to fix build_image failure by syslinux on old branches.

BUG=chromium:953993
TEST=precq is happy

Change-Id: I8459371fce2ba6386362eb57ec8268687d26b976
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/overlays/portage-stable/+/1589395
Reviewed-by: Hung-Te Lin <hungte@chromium.org>
Commit-Queue: Hung-Te Lin <hungte@chromium.org>
Tested-by: Hung-Te Lin <hungte@chromium.org>
4 files changed