our path resolver doesn't support converting to an external path while inside the chroot (which is understandable as once we're inside the chroot, we only see the / path as the chroot itself).

we've been exporting the EXTERNAL_TRUNK_PATH variable to the root of the checkout:
  EXTERNAL_TRUNK_PATH=/usr/local/src/chromiumos
but this doesn't help with the chroot itself as it might be 'chroot' or 'foo' or something else the user has named.

we should formalize this interface and add support for the chroot path, and move it all to the cros_sdk python code.  suggested flow:
  CROS_EXTERNAL_CHECKOUT_PATH=/usr/local/src/chromiumos
  CROS_EXTERNAL_CHROOT_PATH=/usr/local/src/chromiumos/chroot