CHROMIUM: Add config_version sysfs entry

Create a sysfs entry to coordinate configuration changes using poll.
This change is needed for the container watcher daemonset to coordinate
state with the service as it may go down or the VM might migrate
attaching to a different instance.

BUG=b/128676538
TEST=Created new tests and ran existing ones.

Signed-off-by: Thomas Garnier <thgarnie@chromium.org>
Change-Id: Icc00f2ef642e9bb06d64ee8c3de3fa1e3af59fe4
Reviewed-on: https://chromium-review.googlesource.com/1531537
Reviewed-by: Dmitry Torokhov <dtor@chromium.org>
(cherry picked from commit f5398cdca470cebc3f79dc8a2c1853bcd178c289)
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/kernel/+/1553070
Reviewed-by: Robert Kolchmeyer <rkolchmeyer@google.com>
Reviewed-by: Guenter Roeck <groeck@chromium.org>