kbuild: sync top Makefile with Linux 3.18-rc1

kbuild: sync top Makefile with Linux 3.18-rc1

Since Linux 3.15, relative path feature and related fixes,
cleanups have been merged to the top Makefile.

The relative path feature looks stable enough, so let's import it
to U-Boot along with various cleanups.

Commits imported from Linux (some need adjustment) are:

[1] commit 7e1c04779efd by Michal Marek
  kbuild: Use relative path for $(objtree)

[2] commit 890676c65d69 by Michal Marek
  kbuild: Use relative path when building in the source tree

[3] commit 9da0763bdd82 by Michal Marek
  kbuild: Use relative path when building in a subdir of the source tree

[4] commit c2e28dc975ea by Michal Marek
  kbuild: Print the name of the build directory

[5] commit 066b7ed95580 by Michal Marek
  kbuild: Do not print the build directory with make -s

[6] commit 3f1d9a6cec01 by Michal Marek
  kbuild: make -s should be used with kernelrelease/kernelversion/image_name

[7] commit 7ff525712acf by Masahiro Yamada
  kbuild: fake the "Entering directory ..." message more simply

[8] commit 745a254322c8 by Masahiro Yamada
  kbuild: use $(Q) for sub-make target

[9] commit aa55c8e2f7a3 by Masahiro Yamada
  kbuild: handle C=... and M=... after entering into build directory

[10] commit ab7474ea5361 by Borislav Petkov
  Kbuild: Ignore GREP_OPTIONS env variable

To use relative path feature, tools/Makefile and scripts/Makefile.autoconf
must be tweaked.

Signed-off-by: Michal Marek <mmarek@suse.cz>
Signed-off-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>