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/linux-headers-5.10.41/linux-headers-5.10.41.sh

@@ -3,14 +3,6 @@
 #
 # SPDX-License-Identifier: GPL-3.0-or-later
 
-src_unpack() {
-    tarball="${DISTFILES}/linux-headers-5.10.41.tar.gz"
-    tar -xzf "${tarball}" "${dirname}/scripts"
-    tar -xzf "${tarball}" "${dirname}/include"
-    tar -xzf "${tarball}" "${dirname}/arch/x86/include"
-    tar -xzf "${tarball}" "${dirname}/arch/x86/entry"
-}
-
 src_prepare() {
     default
 

sysa/linux-headers-5.10.41/sources

@@ -0,0 +1 @@
+https://mirrors.kernel.org/pub/linux/kernel/v5.x/linux-5.10.41.tar.gz 84d2079a20ba32f5e2d5bc79a5dcb1de94d0176c67d75d5a20d533ea6c90d691 linux-5.10.41.tar.gz linux-5.10.41/scripts linux-5.10.41/include linux-5.10.41/arch/x86/include linux-5.10.41/arch/x86/entry