gbmc-load-dtbo: Add package

Add a common mechanism for loading dtbo files as a service. This ensures
that the dtbocfg out of tree module is loaded into the kernel and then
loads the dtbo.

Tested: Booted on a BMC that has overlays in the rootfs that can be
loaded and have them loaded at bootup and later at runtime to verify
devices are probed as expected.

Google-Bug-Id: 540953723
Change-Id: I473a77582d9aa34adeb99a9f9d125b67720209e0
Signed-off-by: William A. Kennington III <wak@google.com>