We'll soon have other output besides the images, having a single directory
for all output makes sense.

The upstream builds always use LINUX_RELEASE=1. By setting it to 2 we
ensure that our modules are always preferred as long as our kernel version
is at least as high as the upstream one.

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.

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

mpc85xx and x86 are unchanged since rc3.

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.

Closes #63