kbuild: move extra gcc checks to scripts/Makefile.extrawarn

This commit was imported from Linux Kernel:
commit a86fe353 written by me.

W=... provides extra gcc checks.

Having such code in scripts/Makefile.build results in the same flags
being added to KBUILD_CFLAGS multiple times becuase
scripts/Makefile.build is invoked every time Kbuild descends into
the subdirectories.

Since the top Makefile is already too cluttered, this commit moves
all of extra gcc check stuff to a new file scripts/Makefile.extrawarn,
which is included from the top Makefile.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Series-to: uboot
Cover-letter:
Kbuild: sync with Linux 3.16-rc1

Kbuild-related update for Linux 3.16 was merge into the upstream:
Some improvements, refactoring, coding-style fixes, etc.

This series import them to U-Boot.

Sync with commit 6d87c225f5

END
kbuild: sync mixed targets handling with Linux 3.16-rc1

"make %_config all" was supported for the first time in U-Boot:
  commit 53bca5ab
  kbuild: support simultaneous board configuration and "make all"

Surprisingly it had not been working in Linux Kernel for a long time.

So I sent back the patch to the Linux Kbuild community and it was
accepted with a little code improvement, at commit 9319f453.

Now, you can do "make defconfig all" or "make %_defconfig all"
in Linux too.

This commit updates some scripts to fill the code-diff
between Linux and U-Boot.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
cosmetic: kbuild: clean-up coding style (sync with Linux 3.16-rc1)

Import the following trivial commits from Linux v3.16-rc1:

 bb66fc6 kbuild: trivial - use tabs for code indent where possible
 7eb6e34 kbuild: trivial - remove trailing empty lines
 3fbb43d kbuild: trivial - fix comment block indent
 38385f8 kbuild: trivial - remove trailing spaces

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
kbuild: Use relative path when possible

Import 3 useful commits by Michal Marek <mmarek@suse.cz>
from Linux 3.16-rc1.

  commit 9da0763bdd82572be243fcf5161734f11568960f
  kbuild: Use relative path when building in a subdir of the source tree

  commit 890676c65d699db3ad82e7dddd0cf8fb449031af
  kbuild: Use relative path when building in the source tree

  commit 7e1c04779efd51154baf652e653ceb24ce68939b
  kbuild: Use relative path for $(objtree)

mkconfig and tools/Makefile must be adjusted to use relative path
for srctree.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
scripts: objdiff: sync with Linux 3.16-rc1

Import scripts/objdiff improvement from Linux v3.16-rc1, which
consists of 6 commits written by me.

  commit 7fa0e6db3cedc9b70d68a4170f1352e2b1aa0f90
  scripts: objdiff: support directories for the augument of record command

  commit 8ac28bee76eec006aac5ba5c418878a607d53a9b
  scripts: objdiff: fix a comment

  commit 8b5d0f20d64f00ffd5685879f8eb3659379f5aaa
  scripts: objdiff: change the extension of disassembly from .o to .dis

  commit 18165efa8203a34d82f60a1831ea290e7304c654
  scripts: objdiff: improve path flexibility for record command

  commit 1ecc8e489abfdaa6d8d1689f7ff62fdf1adda30c
  scripts: objdiff: remove unnecessary code

  commit 5ab370e91af70d5f1b1dbaec78798a2ff236a2d5
  scripts: objdiff: direct error messages to stderr

  commit fd6e12423311697860f30d10398a0f9eb91977d2
  scripts: objdiff: get the path to .tmp_objdiff more simply

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>