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.

More information on how to get correct image file naming.