vendor_init: Create vendor_init exec env for msmnile bootdevice setup

Create vendor_init (transitioned from init) environment for running
vendor scripts and allow vendor toolbox execution.

Tag temporary, msmnile-specific bring-up getbootdevice vendor script
to run in this vendor_init environment.

Change-Id: Id5fe55c8a6b12468a372892120c8af42aff51a70