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/krb5/package.py

@@ -12,7 +12,7 @@ class Krb5(AutotoolsPackage):
     homepage = "https://kerberos.org"
     url      = "https://kerberos.org/dist/krb5/1.16/krb5-1.16.1.tar.gz"
 
-    version('1.16.1', '848e9b80d6aaaa798e3f3df24b83c407')
+    version('1.16.1', sha256='214ffe394e3ad0c730564074ec44f1da119159d94281bbec541dc29168d21117')
 
     depends_on('bison', type='build')
     depends_on('openssl')