ar71xx: disable building u-boot for ZyXEL devices

The u-boot package has some issues causing its build to fail under certain
circumstances. As we don't support the ZyXEL devices it's used for anyways, just
don't build it anymore.

targets/ar71xx-generic/config

 CONFIG_TARGET_ar71xx=y
 CONFIG_TARGET_ROOTFS_JFFS2=n
+# CONFIG_PACKAGE_uboot-ar71xx-nbg460n_550n_550nh is not set