Not synced completely.

Just cherry-picking low-hanging fruits.

  0b35786d77ba  kbuild: call make once for all targets when O=.. is used
  5e54d5e5fbc1  kbuild: kill EXTRA_ARFLAGS
  836caba77c29  kbuild: kill backward compatibility checks
  3156fd0529b5  kbuild: fix some minor typoes
  b8b0618cf6fa  kbuild: remove extra ifdef/endif of top Makefile
  16f890988114  kbuild: Remove reference to uninitialised variable
  9319f4539c18  kbuild: support simultaneous "make %config" and "make all"
  9d5db8949f1e  scripts/Makefile.clean: clean also $(extra-m) and $(extra-)
  a4954fd7724c  kbuild: remove obj-n and lib-n handling
  371fdc77af44  kbuild: collect shorthands into scripts/Kbuild.include
  a29b82326ed4  kbuild: Remove duplicate $(cmd) definition in Makefile.clean
  a1e7b7bb1ab5  Makefile: sort list of defconfig targets in make help output
  34948e0bbf98  kbuild: Drop support for clean-rule
  4218affdf57f  kbuild: remove warning about "make depend"
  9c8fa9bc08f6  kbuild: fix if_change and friends to consider argument order
  39a33ff80a25  kbuild: remove cc-option-align
  a7f924190924  kbuild: add %.dtb.S and %.dtb to 'targets' automatically
  54a702f70589  kbuild: mark $(targets) as .SECONDARY and remove .PRECIOUS markers
  9564a8cf422d  Kbuild: fix # escaping in .cmd files for future Make
  bd412d81b7ea  kbuild: .PHONY is not a variable, but PHONY is
  6916162c7308  kbuild: remove duplicated comments about PHONY
  d6c6ab93e17f  kbuild: remove deprecated host-progs variable
  7d0ea2524202  kbuild: use 'else ifeq' for checksrc to improve readability
  3f80babd9ca4  kbuild: remove unused cc-fullversion variable
  bd352a739fde  kbuild: remove unused baseprereq
  b421b8a6cb87  kbuild: remove unused archmrproper

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>