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/flang/package.py

@@ -17,8 +17,8 @@ class Flang(CMakePackage):
     git      = "https://github.com/flang-compiler/flang.git"
 
     version('develop', branch='master')
-    version('20180921', '4440ed5fdc390e4b7a085fb77b44ac54')
-    version('20180612', '62284e26214eaaff261a922c67f6878c')
+    version('20180921', sha256='f33bd1f054e474f1e8a204bb6f78d42f8f6ecf7a894fdddc3999f7c272350784')
+    version('20180612', sha256='6af858bea013548e091371a97726ac784edbd4ff876222575eaae48a3c2920ed')
 
     depends_on('llvm@flang-develop', when='@develop')
     depends_on('llvm@flang-20180921', when='@20180921 target=x86_64:')