Run apt-get clean after package install

- Saves about 200MB in Excelsior dev build

Change-Id: I6c5654bf86920efca236b23b6706e0789d1869ff
diff --git a/rootfs.mk b/rootfs.mk
index d3937d6..558c0c0 100644
--- a/rootfs.mk
+++ b/rootfs.mk
@@ -160,6 +160,8 @@
 	sudo chroot $(ROOTFS_DIR) bash -c 'apt-get upgrade -y'
 	$(LOG) rootfs patch bsp finished
 
+	sudo chroot $(ROOTFS_DIR) bash -c 'apt-get clean'
+
 # TODO(jtgans): Remove these when rapture is updated. Until then keeping the local repo
 # is the only way of installing locally built packages on device.
 # ifeq ($(FETCH_PACKAGES),false)