Split out base squashfs.exclude

Variants no longer need to specify the full contents of the
`squashfs.exclude` file.  The "base" file in the source directory gets
combined with the one from the config directory (if it exists).

Makefile

@@ -53,11 +53,17 @@ $(O)/.ready: \
 	./build-rootfs.sh
 	touch $(O)/.ready
 
+$(O)/squashfs.exclude: \
+		squashfs.exclude \
+		$(shell test -f $(CONFIGDIR)/squashfs.exclude && echo $(CONFIGDIR)/squashfs.exclude)
+	mkdir -p $(O)
+	sort $^ > $@
+
 $(IMAGESDIR)/rootfs.squashfs: \
 		build-squashfs.sh \
-		$(CONFIGDIR)/squashfs.exclude \
+		$(O)/squashfs.exclude \
 		$(O)/.ready
-	./build-squashfs.sh "$(IMAGESDIR)"
+	./build-squashfs.sh "$(O)" "$(IMAGESDIR)"
 
 squashfs: $(IMAGESDIR)/rootfs.squashfs