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

This fixes `make update` not working correctly when GLUON_SITEDIR is set
manually.

This adds the new macro GluonProfileFactorySuffix which can be used to override
the factory images suffix ".bin". When the suffix is set to the empty string
Gluon will assume that there are no factory images.

This fixes targets like toolchain/clean.

We were missing the luci packages...

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

Otherwise, `make cleanall` fails...

Explicitly re-build prepare-tmpinfo after the feeds have been installed, as it
isn't done automatically anymore since we have the gluon-tools target.