make: rootfs: Support customize.sh

Projects can now include a `customize.sh` script.  If present, this
script will be run after the root filesystem hierarchy has been created,
but before the image is captured.  This allows projects to apply
additional customizations beyond adding or removing files.

Makefile

@@ -57,8 +57,10 @@ $(O)/.ready: \
 		$(O)/semanage.mods \
 		$(shell find overlay -type f) \
 		$(shell find $(CONFIGDIR)/overlay -type f 2>/dev/null) \
+		$(wildcard $(CONFIGDIR)/customize.sh) \
 		$(O)/.built
 	./build-rootfs.sh "$(O)"
+	if [ -x "$(CONFIGDIR)"/customize.sh ]; then "$(CONFIGDIR)"/customize.sh "$(O)"; fi
 	touch $(O)/.ready
 
 $(O)/squashfs.exclude: \