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/libffi/package.py

@@ -13,9 +13,9 @@ class Libffi(AutotoolsPackage):
     run time."""
     homepage = "https://sourceware.org/libffi/"
 
-    version('3.2.1', '83b89587607e3eb65c70d361f13bab43',
+    version('3.2.1', sha256='d06ebb8e1d9a22d19e38d63fdb83954253f39bedc5d46232a05645685722ca37',
             url="https://www.mirrorservice.org/sites/sourceware.org/pub/libffi/libffi-3.2.1.tar.gz")
-    # version('3.1', 'f5898b29bbfd70502831a212d9249d10',url =
+    # version('3.1', md5='f5898b29bbfd70502831a212d9249d10',url =
     # "ftp://sourceware.org/pub/libffi/libffi-3.1.tar.gz") # Has a bug
     # $(lib64) instead of ${lib64} in libffi.pc