Merge git://git.kernel.org/pub/scm/linux/kernel/git/sam/kbuild-next

* git://git.kernel.org/pub/scm/linux/kernel/git/sam/kbuild-next:
  kbuild: ignore a few files in headers_check
  kbuild: add checks for include of linux/types in userspace headers
  kbuild: drop debugging leftover in tags.sh
  kbuild: document environment variables
  kbuild: make *config usage docs
  kbuild: disable sparse warning "returning void-valued expression"
  kbuild: in headers_install autoconvert asm/inline/volatile to __xxx__
  kbuild: check for leaked CONFIG_ symbols to userspace
  headers_check.pl: disallow extern's
  kconfig: improve error messages for bad source statements
  kconfig: struct property commented
  kconfig: add comments to symbol flags
  kconfig: explain symbol value defaults
  m68k: fix recursive dependency in Kconfig