Modify filesystem-layout hook to check for edgeos

- On EdgeOS, we need to create /export/hda3 and make some other
  directories writable.
- However, this hook disallows that because these are nonstandard root
  directories.
- This changeset allows ebuilds to declare themselves as EdgeOS ebuilds,
  and if they do allows them to leave /export and /users in the
  filesystem root.
- More context:
  https://groups.google.com/a/google.com/g/chromeos-build-discuss/c/cjETHlXhUyk/m/6hH8mVGpAAAJ

BUG=b/287334438
TEST=Built an ebuild with this
RELEASE_NOTE=None

Change-Id: Ib10143f4bf890644aed2cad9c51fae2b8b2f4b33
Reviewed-on: https://cos-review.googlesource.com/c/third_party/platform/crosutils/+/60642
Reviewed-by: Robert Kolchmeyer <rkolchmeyer@google.com>
Reviewed-by: Kirill Spitsyn <ksp@google.com>
Tested-by: Cusky Presubmit Bot <presubmit@cos-infra-prod.iam.gserviceaccount.com>