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/ca-certificates-3.78/ca-certificates-3.78.sh

@@ -2,8 +2,6 @@
 #
 # SPDX-License-Identifier: GPL-3.0-or-later
 
-urls="http://ftp.mozilla.org/pub/security/nss/releases/NSS_3_78_RTM/src/nss-3.78.tar.gz"
-
 src_compile() {
     cp -a nss/lib/ckfw/builtins/certdata.txt .
     mk-ca-bundle -n -s ALL -m

sysc/ca-certificates-3.78/sources

@@ -0,0 +1 @@
+http://ftp.mozilla.org/pub/security/nss/releases/NSS_3_78_RTM/src/nss-3.78.tar.gz f455f341e787c1167328e80a84f77b9a557d595066dda6486a1874d72da68800