dts, kbuild: Factor out dtbs install rules to Makefile.dtbinst

Move dtbs install rules to Makefile.dtbinst. This change is needed to
implement support for dts vendor subdirs. The change makes Makefiles
easier and smaller as no longer the dtbs_install rule needs to be
defined. Another advantage is that install goals are not encoded in
targets anymore (%.dtb_dtbinst_).

Signed-off-by: Robert Richter <rrichter@cavium.com>
diff --git a/scripts/Kbuild.include b/scripts/Kbuild.include
index 65e7b08..5374b1b 100644
--- a/scripts/Kbuild.include
+++ b/scripts/Kbuild.include
@@ -179,6 +179,12 @@
 # $(Q)$(MAKE) $(modbuiltin)=dir
 modbuiltin := -f $(srctree)/scripts/Makefile.modbuiltin obj
 
+###
+# Shorthand for $(Q)$(MAKE) -f scripts/Makefile.dtbinst obj=
+# Usage:
+# $(Q)$(MAKE) $(dtbinst)=dir
+dtbinst := -f $(if $(KBUILD_SRC),$(srctree)/)scripts/Makefile.dtbinst obj
+
 # Prefix -I with $(srctree) if it is not an absolute path.
 # skip if -I has no parameter
 addtree = $(if $(patsubst -I%,%,$(1)), \