Add postinst/prerm scripts and service for u-boot

- Add a uboot-imx systemd service: this checks the bootloader on eMMC,
and replaces it with the packages version if they differ.
- Enable the service during postinst, and if the boot partition exists
(on device, as opposed to a chroot), go ahead and start the service as
well.
- Disable the service in prerm.

Change-Id: I539726973036a34897770f7df004b36d3656deee