Removes cyclic dependency on spack.config. (#2121)

Merge #2030 added a cyclic dependency between the Cray platform needing
to read a `targets.yaml` config file and `config.py` needing to get the
platform names.

This commit removes the cyclic dependency in favor of the more general
config scheme. It also removes the now functionless `targets.yaml`
config file. This breaks 'frontend' targets on the Cray platform but
all architecture targets, including the frontend, that are provided by
CrayPE are added to the Platform anyway so users can be explicit about
the architecture targeted by the Cray compiler wrappers:

```
spack spec libelf arch=cray-CNL-frontend
```

becomes

```
spack spec libelf arch=cray-CNL-mc8         # on an XK7 or
spack spec libelf arch=cray-CNL-sandybridge # on an older XC30, etc..
```

The only way the 'frontend' target can be defined after this commit is
through target environment variables.