init: refactor: add support for doing early coldboot

We don't want to spend time creating devices that are unncessesary
during early (init first-stage) mount. So, refactor the devices code
tha allows us to call into coldboot and has the

- ability to only create devices that are specified by the caller
- ability to stop coldboot cycle when all devices that the caller is
interested in
- ability to run coldboot for a specific syspath
- ability to run ueventd code unmodified

Test: Tested boot on angler, sailfish

Change-Id: Id8f3492380696760414eadc20d624d300c904f8e
Signed-off-by: Sandeep Patil <sspatil@google.com>