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

sysc/binutils-2.38/binutils-2.38.sh

@@ -4,7 +4,6 @@
 #
 # SPDX-License-Identifier: GPL-3.0-or-later
 
-urls="https://mirrors.kernel.org/gnu/binutils/binutils-2.38.tar.xz"
 
 src_prepare() {
     default

sysc/binutils-2.38/sources

@@ -0,0 +1 @@
+https://mirrors.kernel.org/gnu/binutils/binutils-2.38.tar.xz e316477a914f567eccc34d5d29785b8b0f5a10208d36bbacedcc39048ecfe024