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 reverts commit 59ea630e.

This shouldn't affect compatiblity with kernel modules and will allow us to
find out under which hypervisor we're running.

No autoupdater support yet, but that will follow.

x86-generic is still marked as broken as fastd v16 doesn't work with uClibc on
x86. This will be fixed with fastd v17.