Change sources getting method.

- Rather than defining the urls where they are gotten (python sysa,
  python sysc, inside sysc), a spec file is now used that is easily
  interpretable and tool-independent.
- This is interpreted by rootfs.py and inside sysc.
- This is also used to make sources available and extract sources.
- Manual dirname selection is no longer required as is tarball renaming
  upon download - all of this is handled automatically.

Fixes #188

sysa/mes-0.24/mes-0.24.kaem

@@ -14,6 +14,10 @@ MES_STACK=6000000
 MES=${bindir}/mes-m2
 libdir=${MES_PREFIX}/lib
 
+# Check tarball checksums
+checksum-transcriber sources
+sha256sum -c sources.SHA256SUM
+
 # Unpack
 mkdir src build
 cd src