kbuild: support simultaneous board configuration and "make all"

This commit fixes two problems:

[1] We could not do board configuration and "make all"
    in one command line.

For example, the following did not work as we expect:
  $ make sandbox_config all
  Configuring for sandbox board...
  make: Nothing to be done for `all'.

[2] mixed-target build did not work with -j option

For example, the following did not work:
  $ make -j8 sandbox_config u-boot
  Makefile:481: *** "System not configured - see README".  Stop.
  make: *** [u-boot] Error 2
  make: *** Waiting for unfinished jobs....
  Configuring for sandbox board...

Going forward, we can do
  $ make -j8 sandbox_config all

This is the same as
  $ make sandbox_config
  $ make -j8
Signed-off-by: NMasahiro Yamada <yamada.m@jp.panasonic.com>

Makefile

@@ -428,8 +428,16 @@ ifeq ($(mixed-targets),1)
 # We're called with mixed targets (*config and build targets).
 # Handle them one by one.
 
-%:: FORCE
-	$(Q)$(MAKE) -C $(srctree) KBUILD_SRC= $@
+PHONY += $(MAKECMDGOALS) build-one-by-one
+
+$(MAKECMDGOALS): build-one-by-one
+	@:
+
+build-one-by-one:
+	$(Q)set -e; \
+	for i in $(MAKECMDGOALS); do \
+		$(MAKE) -f $(srctree)/Makefile $$i; \
+	done
 
 else
 ifeq ($(config-targets),1)