A platform to generically cover all Cray systems is introduced to avoid having
specific platforms for each of XK (XE, XT), XC, and future systems using CrayPE
and CNL.
The platform searches for 'front_end' and 'back_end' targets, in order, from:
* Environment variables 'SPACK_FRONT_END' and 'SPACK_BACK_END'
* A spack configuration file 'targets.yaml'
* Parsing `/etc/bash.bashrc.local` for first loaded CrayPE CPU target.
If a back_end target is not found through one of these methods, an exception is
raised. Otherwise, the back_end is set as the default target. The shell init
script search is based on recommendations in Crayports case #144359.
No automagic way of determining front_end targets has been found (to date) so if
a front_end is not specified through configuration, it is ignored which should
leave a spack instance in a consistant state.
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