The new options allow specifying custom repositories and are evaluated
in an upgrade script, not during build.

This also takes us past the OpenWrt 15.05 final release.

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.

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

Also add some patches from OpenWrt trunk and use the CT ath10k firmware.

mpc85xx and x86 are unchanged since rc3.

Updating OpenWrt accidentally enabled this option.

They aren't used by default and can simply be installed through opkg if needed.

This allows us to use separate .config files per target. Also, don't use
OpenWRT's feeds script anymore, but symlink the feeds ourselves.

Also, more cleanup.

Introduce a new value "opkg_repo" in the site configuration.
The value should be a single domain name,
e.g. "http://opkg.services.ffki/attitude_adjustment/12.09/%S/packages".

This *should* fix the build on non-Linux systems like MacOS X.