chromeos-init-systemd: Allow overriding OEM device.

This is useful if you want to share the same persistant disk between two
VMs for configuration or misc storage. Also, if you want to use another
location for storing OEM data (like an encrypted folder using dm-crypt).

BUG=b/209419163
TEST=systemd mounts /usr/share/oem to the specified device, otherwise
falls back to /dev/sda8.
RELEASE_NOTE=none

Change-Id: I838fcfe031df3ed81e885fc5b50d26867a3f3d36
Reviewed-on: https://cos-review.googlesource.com/c/cos/overlays/board-overlays/+/27241
Tested-by: Cusky Presubmit Bot <presubmit@cos-infra-prod.iam.gserviceaccount.com>
Reviewed-by: Roy Yang <royyang@google.com>
2 files changed