Makefile: fix dependency problem with parallel builds

Parallel builds (using "make -jN") would occasionally fail with error
messages like
	include/autoconf.mk:212: *** missing separator.  Stop.
Line numbers and affected boards were changing. Obviously some
Makefiles included autoconf.mk while it was still being written to.
As a fix, we now write to a temporary file first and then rename it,
so that it is really ready to use as soon as it appears.

Signed-off-by: Wolfgang Denk <wd@denx.de>

Makefile

@@ -463,7 +463,8 @@ $(obj)include/autoconf.mk: $(obj)include/config.h
 	set -e ; \
 	: Extract the config macros ; \
 	$(CPP) $(CFLAGS) -DDO_DEPS_ONLY -dM include/common.h | \
-		sed -n -f tools/scripts/define2mk.sed > $@
+		sed -n -f tools/scripts/define2mk.sed > $@.tmp && \
+	mv $@.tmp $@
 
 sinclude $(obj)include/autoconf.mk.dep