usign has pulled in new dependencies, and OpenWrt doesn't like the command
`make tools/prepare` in a clean build.

This will fix the early preparation happening much too often and also fixes
early preparation after `make clean`.

This broke due to the Makefile changes for building the module repo.

`make modules` will generate a opkg repository containing kernel modules
matching the kernel of the Gluon images.

As the Gluon kernel won't be compatible with the OpenWrt upstream anymore,
we don't need the vermagic hack anymore.

Add an optional third argument to the GluonProfile macro while contains the
OpenWrt profile name. This allows creating different Gluon profiles (with
different image names etc.) from the same OpenWrt profile.

The file targets/$GLUON_TARGET/config becomes optional, as many targets
only used it to set the board and subtarget.

Also fix targets without subtarget.

Fixes build for brcm2708 targets

The oldpackages repo has been dropped for Chaos Calmer.

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.