Remove boot-targets, have all depend on rootfs.img

- Seems like our dependency graph is actually not broken these days, and
there's no funny interdependence between u-boot and the kernel build
since they're done in pbuilder. It's now possible to run a build from
scratch just depending on rootfs.img, so clean up the messiness of

Change-Id: Iaff45157f44a908c594ac38b6066344f9c516bac
2 files changed