stage1/rootfs: various small Makefile dependency fixups

 * usr/Makefile lacked dependency on manifest.d
 * aggregate/Makefile lacked dependency on discrete components:
   - Introduce aggregate/install.d registry of ordered component installers
   - Add Makefile dependency on aggregate/install.d/*
   - Amend discrete component's Makefiles to register their installation
     as aggregate/install.d/$prio$component scripts sourced by aggregate.sh

 These changes cause aggregate to occur when any discrete component is built
 in an incremental rootfs build.  Otherwise discrete components would be built
 without getting pulled into a (re)built rootfs.