The file targets/$GLUON_TARGET/config becomes optional, as many targets
only used it to set the board and subtarget.

Also fix targets without subtarget.

With a backported patch from the OpenWrt trunk, it is now easy to generate an
equivalent configuration using CONFIG_ALL_KMODS.

The build will take a bit longer because all kernel module packages are actually
built even when they are not included in the image, but adding new targets
becomes a lot easier.

Also, related documentation updates and fixes.

This still needs some work:

- there's no factory image causing `make images` to fail
- VoCore can not support BSS and IBSS at the same time
- multi-BSS mode (e.g. mesh + BSS) works if the macs only
  differ in the last 3 bits. Gluon expects to choose both MACs freely,
  though, so after flashing the image one should reset the wifi MACs.
  This is further complicated as VoCores have their MACs assigned
  without gaps making collisions likely.
- there are no buttons nor ethernet ports (without the dock, that is),
  so config-mode will not be possible as is