build: Only copy-paste ubinize.cfg from target/image if it exists

For some images, having a single ubinize.cfg does not or will not
make sense. If that file does not exist, a build rule can
conditionally place it where ubinize expects it, without the copy
failing because it cannot find ubinize.cfg.
ar71xx: Call Image/Build/$foo for ubi and ubifs generation

This patch makes sure appropriate targets (rules?) get called after
generation of ubi or ubifs images so that device specific steps can
be taken to finalize them.
ar71xx: Add UBI/UBIFS options to WNDR4300 profile.
ar71xx: Add ubi/ubifs to kernel config.
ar71xx: Add ubifs to front of rootfs command

If ubifs is not in front of the other two, the generated images do
not boot, because the ubifs driver is not (yet?) loaded when the
kernel is ready to mount the filesystem.
ar71xx: Enable ubi/ubifs defines for NetgearNAND.

This is basically copy-pasted from proposed patches by Stefan Agner,
https://github.com/falstaff84/openwrt/commit/995d680c3d7a2edd6c41e4820d2adee5b0c13ddc,
but with some slight modifications to separate the steps a little.