aboot: publish serialno for getvar

Change-Id: I3b494c98d59ff5c1a9d536ce7191096896f9f353
diff --git a/app/aboot/aboot.c b/app/aboot/aboot.c
index 19d51a8..6cf6264 100644
--- a/app/aboot/aboot.c
+++ b/app/aboot/aboot.c
@@ -1276,6 +1276,7 @@
 	fastboot_register("oem device-info", cmd_oem_devinfo);
 	fastboot_publish("product", TARGET(BOARD));
 	fastboot_publish("kernel", "lk");
+	fastboot_publish("serialno", sn_buf);
 	partition_dump();
 	sz = target_get_max_flash_size();
 	fastboot_init(target_get_scratch_address(), sz);