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.

Don't set ar71xx-generic target by default, generate manifest for all built
targets.

Includes a profile for the NETGEAR WNDR3700v4 and WNDR4300v1