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/libffi-3.3/libffi-3.3.sh

@@ -2,8 +2,6 @@
 #
 # SPDX-License-Identifier: GPL-3.0-or-later
 
-urls="https://github.com/libffi/libffi/releases/download/v3.3/libffi-3.3.tar.gz"
-
 src_prepare() {
     find . -name '*.info*' -delete
 

sysc/libffi-3.3/sources

@@ -0,0 +1 @@
+https://github.com/libffi/libffi/releases/download/v3.3/libffi-3.3.tar.gz 72fba7922703ddfa7a028d513ac15a85c8d54c8d67f55fa5a4802885dc652056