improve modalias building

For one, there's no point in the respective pieces to be rebuilt
unconditionally on each and every rebuild.

Second there's no need to invent a custom rule for generating the .s
file from the .c source - we can simply use the generic rule here.

And finally, $(obj) should be used to refer to files in the build tree
(rather than spelling out the subdirectory).

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>

scripts/mod/Makefile

@@ -29,15 +29,10 @@ define cmd_offsets
 	 echo "#endif" ) > $@
 endef
 
-# We use internal kbuild rules to avoid the "is up to date" message from make
-scripts/mod/devicetable-offsets.s: scripts/mod/devicetable-offsets.c FORCE
-	$(Q)mkdir -p $(dir $@)
-	$(call if_changed_dep,cc_s_c)
+$(obj)/$(devicetable-offsets-file): $(obj)/devicetable-offsets.s
+	$(call if_changed,offsets)
 
-$(obj)/$(devicetable-offsets-file): scripts/mod/devicetable-offsets.s
-	$(call cmd,offsets)
-
-targets += $(devicetable-offsets-file)
+targets += $(devicetable-offsets-file) devicetable-offsets.s
 
 # dependencies on generated files need to be listed explicitly