update ~/trunk usage to ~/chromiumos

Everyone should have ~/chromiumos now, so update the code to use that
by default to find the CrOS source checkout instead of ~/trunk.

BUG=None
TEST=CQ passes

Change-Id: Ib12a0a7950fdf2212fb1af7d8d0b7ea79d6d9f3b
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosutils/+/3384294
Reviewed-by: Alex Klein <saklein@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
2 files changed