update_chroot: Add backtrack argument.

Add --backtrack to allow configuring emerge's --backtrack.
Also fixed a few shellcheck variable quoting/braces lints.

BUG=b:237827425
TEST=CQ

Change-Id: I896856a840d5e9935deee83c9c389f1c1f62f049
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosutils/+/3792205
Commit-Queue: Mike Frysinger <vapier@chromium.org>
Auto-Submit: Alex Klein <saklein@chromium.org>
Commit-Queue: Alex Klein <saklein@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Tested-by: Alex Klein <saklein@chromium.org>
1 file changed