kbuild: show defconfig subdirs in make help

PowerPC will start moving board defconfigs into subarch-specific
subdirs soon.  "make help" currently does not look in subdirs to
find the defconfigs to show.  This is partially a good thing,
since there are way too many defconfigs for one list.

This patch makes the main "make help" display something like

  help-40x         - Show 40x-specific targets
  help-44x         - Show 44x-specific targets
  help-boards      - Show all of the above

and wires up stuff so those new help-* commands actually work.

[sam: fixed it up to display x86 defconfigs too]
Cc: Josh Boyer <jwboyer@linux.vnet.ibm.com>
Signed-off-by: Segher Boessenkool <segher@kernel.crashing.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>