support installing Chromium OS chroot in another chroot

The detection of whether we are inside the Chromium OS build chroot is
making a false positive when run inside Crouton which is itself a
Ubuntu chroot.
Let's filter out all Debian derivative chroot and check only for a
genuine Chromium OS chroot.

BUG=none
TEST=on Pixel in dev mode, run cros_sdk in the Crouton chroot.
CQ-DEPEND=I3c143e85c141e2def038fb859c90bea7003fc875

Change-Id: I723fc57805d3891d00a111547f9aaa33231aa9ea
Reviewed-on: https://chromium-review.googlesource.com/188071
Commit-Queue: Vincent Palatin <vpalatin@chromium.org>
Tested-by: Vincent Palatin <vpalatin@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
3 files changed