checksums: use sha256 checksums everywhere

We'd like to use a consistent checksum scheme everywhere so that we can:

    a) incorporate archive checksums into our specs and have a
       consistent hashing algorithm across all specs.

    b) index mirrors with a consistent type of checksum, and not one that
       is dependent on how spack packages are written.

- [x] convert existing md5, sha224, sha512, sha1 checksums to sha256

var/spack/repos/builtin/packages/clfft/package.py

@@ -12,7 +12,7 @@ class Clfft(CMakePackage):
     homepage = "https://github.com/clMathLibraries/clFFT"
     url      = "https://github.com/clMathLibraries/clFFT/archive/v2.12.2.tar.gz"
 
-    version('2.12.2', '9104d85f9f2f3c58dd8efc0e4b06496f')
+    version('2.12.2', sha256='e7348c146ad48c6a3e6997b7702202ad3ee3b5df99edf7ef00bbacc21e897b12')
 
     variant('client', default=True,
             description='build client and callback client')