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>

scripts/mkmakefile

@@ -42,18 +42,11 @@ MAKEARGS += O=\$(if \$(patsubst /%,,\$(makedir)),\$(CURDIR)/)\$(patsubst %/,%,\$
 
 MAKEFLAGS += --no-print-directory
 
-.PHONY: all \$(MAKECMDGOALS)
+.PHONY: __sub-make \$(MAKECMDGOALS)
 
-all	:= \$(filter-out all Makefile,\$(MAKECMDGOALS))
+__sub-make:
+	\$(Q)\$(MAKE) \$(MAKEARGS) \$(MAKECMDGOALS)
 
-all:
-	\$(Q)\$(MAKE) \$(MAKEARGS) \$(all)
-
-Makefile:;
-
-\$(all): all
-	@:
-
-%/: all
+\$(filter-out __sub-make, \$(MAKECMDGOALS)): __sub-make
 	@:
 EOF