Fix config files for out-of-tree building

Several board/<...>/config.mk files include dynamically built (by
the Makefile) config files but used the wrong file name of
	$(TOPDIR)/board/$(BOARDDIR)/config.tmp
instead if the correct
	$(OBJTREE)/board/$(BOARDDIR)/config.tmp

The bug is nasty because the build result is correct for the (normal)
in-tree builds, and because 'sinclude' is used no errors get raised
even for out-of-tree build tests. But out-of-tree builds use an
incomplete and thus usually incorrect configuration...

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