kbuild: remove duplication from modules.order in sub-directories

Currently, only the top-level modules.order drops duplicated entries.

The modules.order files in sub-directories potentially contain
duplication. To list out the paths of all modules, I want to use
modules.order instead of parsing *.mod files in $(MODVERDIR).

To achieve this, I want to rip off duplication from modules.order
of external modules too.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

scripts/Makefile.build

@@ -423,13 +423,10 @@ endif # builtin-target
 #
 # Create commands to either record .ko file or cat modules.order from
 # a subdirectory
-modorder-cmds =						\
-	$(foreach m, $(modorder),			\
-		$(if $(filter %/modules.order, $m),	\
-			cat $m;, echo $m;))
-
 $(modorder-target): $(subdir-ym) FORCE
-	$(Q)(cat /dev/null; $(modorder-cmds)) > $@
+	$(Q){ $(foreach m, $(modorder), \
+	$(if $(filter %/modules.order, $m), cat $m, echo $m);) :; } \
+	| $(AWK) '!x[$$0]++' - > $@
 
 #
 # Rule to compile a set of .o files into one .a file (with symbol table)