Cleanup .boards.depend when using an objtree

.boards.depend was created in the source tree even when calling make
with O=objtree, and distclean O=objtree wouldn't clean it.  Create
.boards.depend in objtree instead as to clean it up properly.
Reported-by: NLoc Minier <loic.minier@linaro.org>
Signed-off-by: NWolfgang Denk <wd@denx.de>

Makefile

@@ -525,8 +525,8 @@ unconfig:
 %_config::	unconfig
 	@$(MKCONFIG) -A $(@:_config=)
 
-sinclude .boards.depend
-.boards.depend:	boards.cfg
+sinclude $(obj).boards.depend
+$(obj).boards.depend:	boards.cfg
 	awk '(NF && $$1 !~ /^#/) { print $$1 ": " $$1 "_config; $$(MAKE)" }' $< > $@
 
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