o When using "make -j<n>", the order in modversions.h could change,
  thus causing unnecessary rebuilds.
o Move a comment in Rules.make which I forgot when I moved the
  associated code.
o When descending into init/ during the final vmlinux link,
  add a '+' do tell make to use its normal "-j" jobserver handling.
o It'd be nice to get the logic between make clean and make mrproper
  clearer. I think make clean should remove all targets used in the
  actual build,
  make mrproper should remove the rest, build-specific files, like
  e.g. the .*.cmd files used to save dependencies and command lines
  and other temporary files.
  A real small step in the direction: Remove .tmpversion/.tmp-export-objs
  at mrproper time.