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/bash-2.05b/bash-2.05b.kaem

@@ -8,6 +8,10 @@
 
 set -ex
 
+# Check tarball checksums
+checksum-transcriber sources
+sha256sum -c sources.SHA256SUM
+
 mkdir build src
 cd build
 

sysa/bash-2.05b/sources

@@ -0,0 +1 @@
+https://mirrors.kernel.org/gnu/bash/bash-2.05b.tar.gz ba03d412998cc54bd0b0f2d6c32100967d3137098affdc2d32e6e7c11b163fe4