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: 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.
ar71xx: Create netgear subtarget and move wndr4300 there

This commit creates a netgear subtarget, and moves the wndr4300
profile into it. Furthermore, the FEATURES are moved to the
subtarget definition. The subtarget is UBI(FS) only, so all
SquashFS/JFFS2 defines are removed in its config-default.

The image Makefile has some small adjustments to ensure the
proper routines get called now that the profile is in a
different subtarget.