runonce: Migrate the runonce service to its own package

This is the start of a bit of sharding out of the aiy-board-tweaks package into
smaller packages that simply depend on the behavior of this script. Should make
maintenance of board-specific packages somewhat easier, since the tweaks package
contained a whole bunch of board specific behaviors.

Change-Id: I132385572f8a8ce9d170ff84daf2fd61ec6472e6