kbuild: add objectify

Use foo := $(call objectify, $(foo)) to prefix $(foo) with $(obj)/ unless
$(foo) is an absolute path.
For now no in-tree users - soon to come.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
diff --git a/scripts/Kbuild.include b/scripts/Kbuild.include
index 9087273..db3c708 100644
--- a/scripts/Kbuild.include
+++ b/scripts/Kbuild.include
@@ -49,6 +49,9 @@
 cmd = @$(if $($(quiet)cmd_$(1)),\
       echo '  $(subst ','\'',$($(quiet)cmd_$(1)))' &&) $(cmd_$(1))
 
+# Add $(obj)/ for paths that is not absolute
+objectify = $(foreach o,$(1),$(if $(filter /%,$(o)),$(o),$(obj)/$(o)))
+
 ###
 # if_changed      - execute command if any prerequisite is newer than 
 #                   target, or command line has changed