Bug#952889: [live-build] sources.list generation code duplication

[jnqnfe@gmail.com]

Package: live-build
Version: 1:20191221
Owner: jnqnfe@gmail.com

there is a significant chunk of code for generating an apt sources.list
file that is duplicated in three different places in the codebase. in
two cases it is identical, in the third it only differs in the
variables used for mirrors and such.

this can very easily be extracted to a common function and would be a
notable improvement in terms of maintainability.

specifically bootstrap_archives holds one copy and chroot_archives the
other two.

patch to be submitted via salsa shortly (will amend commit with bug
number first)