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>