Merge remote-tracking branch 'bonzini/nested-makefiles-3' into staging

* bonzini/nested-makefiles-3: (29 commits)
  build: do not create directories at configure time
  configure: ensure directory exists when creating symlink
  build: compile oslib-obj-y once
  build: limit usage of vpath
  build: libcacard Makefile cleanups
  build: move device tree to per-target Makefile.objs
  build: move per-target hw/ objects to nested Makefile.objs
  build: convert libhw to nested Makefile.objs
  build: move target-independent hw/ objects to nested Makefile.objs
  build: move qga/ objects to nested Makefile.objs
  build: move qapi/ objects to nested Makefile.objs
  build: move slirp/ objects to nested Makefile.objs
  build: move audio/ objects to nested Makefile.objs
  build: move ui/ objects to nested Makefile.objs
  build: move fsdev/ objects to nested Makefile.objs
  build: move net/ objects to nested Makefile.objs
  build: move block/ objects to nested Makefile.objs
  build: adapt qom/Makefile and move it to Makefile.objs
  build: move rules for nesting to Makefile.objs
  build: move other target-*/ objects to nested Makefile.objs
  ...