build: De-duplicate _DIRS before calling mkdir

If the build path is long, the contents of the _DIRS variable can be
very long, since it repeats the same directories very often. In some
cases, this has triggered an "Argument list too long" build error.

Reported-by: Robert Marko <robimarko@gmail.com>
Suggested-by: Eneas U de Queiroz <cotequeiroz@gmail.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name>

src/build.rules

@@ -80,7 +80,7 @@ endif
 _DIRS := $(BUILDDIR)/$(PROJ)
 .PHONY: _make_dirs
 _make_dirs:
-	@mkdir -p $(_DIRS)
+	@mkdir -p $(sort $(_DIRS))
 
 $(BUILDDIR)/$(PROJ)/src/%.o: $(ROOTDIR)src/%.c $(CONFIG_FILE) | _make_dirs
 	$(Q)$(CC) -c -o $@ $(CFLAGS) $<