When update-initramfs is called, clean up the backup files.

This removes the unnecessary vmlinuz.old and initrd.img.old files in the
root folder of the generated image.

scripts/build/chroot_hacks

@@ -67,6 +67,8 @@ esac
 if [ "${LB_INITRAMFS}" != "none" ]
 then
 	Chroot chroot "${UPDATE_INITRAMFS_OPTIONS} update-initramfs -k all -t -u"
+	rm -f chroot/vmlinuz.old
+	rm -f chroot/initrd.img.old
 fi
 
 # We probably ought to use COMPRESS= in a temporary file in