make_chroot: setup crossdev masters correctly

We're putting more eclasses into eclass-overlay/, so make sure it's
listed as a master from the start to find them.  When we run crossdev
later on, it will generate it with the same values.

BUG=b:278264112
TEST=`cros_sdk --delete --create; sudo -E /mnt/host/source/chromite/bin/cros_setup_toolchains --include-boards=arm64-generic`

Change-Id: Ie5c702cda0c37c098e6b0f2ed95fb07b0cd3c034
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crosutils/+/4826252
Tested-by: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Cindy Lin <xcl@google.com>
Reviewed-on: https://cos-review.googlesource.com/c/third_party/platform/crosutils/+/57750
Reviewed-by: Oleksandr Tymoshenko <ovt@google.com>
Reviewed-by: Robert Kolchmeyer <rkolchmeyer@google.com>
Tested-by: Cusky Presubmit Bot <presubmit@cos-infra-prod.iam.gserviceaccount.com>