kbuild: sync mixed targets handling with Linux 3.16

"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>