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.
