kbuild: compute the dtbs_install destination more simply

The 'dtbinst_root' is used to remember the root of the in-kernel dts
directory (i.e. arch/*/boot/dts), but it looks clumsy.

I prefer using two variables 'obj' and 'dst' to track the in-kernel
directory and the install destination, respectively.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
diff --git a/Makefile b/Makefile
index 8533bb6..d32e8c8 100644
--- a/Makefile
+++ b/Makefile
@@ -1258,7 +1258,7 @@
 dtbs_check: dtbs
 
 dtbs_install:
-	$(Q)$(MAKE) $(dtbinst)=$(dtstree)
+	$(Q)$(MAKE) $(dtbinst)=$(dtstree) dst=$(INSTALL_DTBS_PATH)
 
 ifdef CONFIG_OF_EARLY_FLATTREE
 all: dtbs