Otherwise some dependencies aren't respected.

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.

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.