powerpc/boot/dts: Install dts from the right directory

The dts-installed variable is initialised using a wildcard path that
will be expanded relative to the build directory.  Use the existing
variable dtstree to generate an absolute wildcard path that will work
when building in a separate directory.
Reported-by: Gerhard Pircher <gerhard_pircher@gmx.net>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Tested-by: Gerhard Pircher <gerhard_pircher@gmx.net> [against 2.6.32]
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/boot/Makefile

@@ -368,7 +368,7 @@ INSTALL := install
 extra-installed		:= $(patsubst $(obj)/%, $(DESTDIR)$(WRAPPER_OBJDIR)/%, $(extra-y))
 hostprogs-installed	:= $(patsubst %, $(DESTDIR)$(WRAPPER_BINDIR)/%, $(hostprogs-y))
 wrapper-installed	:= $(DESTDIR)$(WRAPPER_BINDIR)/wrapper
-dts-installed		:= $(patsubst $(obj)/dts/%, $(DESTDIR)$(WRAPPER_DTSDIR)/%, $(wildcard $(obj)/dts/*.dts))
+dts-installed		:= $(patsubst $(dtstree)/%, $(DESTDIR)$(WRAPPER_DTSDIR)/%, $(wildcard $(dtstree)/*.dts))
 
 all-installed		:= $(extra-installed) $(hostprogs-installed) $(wrapper-installed) $(dts-installed)