Document changes to hostname_prefix field of site.conf

With this change, there are two OpenWrt config files: one for the packages
actually used by Gluon, and one with all kernel modules enabled (like it was
since we removed the static kernel configs). The config with all kernel modules
is only used to generate the kernel configuration (so it is compatible with the
upstream kernel).

This should bring the build times back to what they were before the static
configs were removed.

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.