Changed default to CPU_TARGET env var. Helps deal with target differences between cori and edison

lib/spack/spack/architectures/cray.py

@@ -6,15 +6,23 @@ class Cray(Architecture):
     priority    = 20
     front_end   = 'sandybridge' 
     back_end    = 'ivybridge'
-    default     = 'ivybridge'
-
+    default     = os.environ['CRAY_CPU_TARGET'] 
+    #default     = 'ivybridge' 
+    
     def __init__(self):
+        ''' Since cori doesn't have ivybridge as a front end it's better
+            if we use CRAY_CPU_TARGET as the default. This will ensure
+            that if we're on a XC-40 or XC-30 then we can detect the target
+        '''
         super(Cray, self).__init__('cray')
+        
         # Back End compiler needs the proper target module loaded.
-        self.add_target('ivybridge', Target('ivybridge','craype-ivybridge'))
+        self.add_target(self.front_end, Target(self.front_end,'craype-' + self.front_end))
+        self.add_target(self.default, Target(self.default,'craype-' + self.default))
         # Could switch to use modules and fe targets for front end
         # Currently using compilers by path for front end.
-        self.add_target('sandybridge', Target('sandybridge'))
+        self.add_target(self.back_end, Target('craype-' + self.back_end))
+        
 
     @classmethod
     def detect(self):