release-source: Generate separate patch for build system changes
release-source: Generate separate patch for build system changes

The previous approach of including the build system changes into every
single patch has several drawbacks:

 - It is susceptible to file timestamps being out of order, i. e.
   sometimes generated files get patched before source files. This
   causes build failures as webpack et al are not available in
   downstream build environments.

 - It makes patches unnecessarily big (due to duplicated patches to
   generated files) and unreadable.

 - It makes patch generation unnecessarily slow, as each new patch
   requires a full build cycle. This previously mis-motivated us in the
   past to merge several upstream patches into one downstream patch
   instead of using clean cherry-picks.

Change and simplify the approach to use the upstream patches verbatim,
and generate a new 0999-Build-system-generated-changes-from-patches.patch
on top of them. This will ensure that build system changes always get
applied after source changes, and thus avoid timestamp sorting problems
that confuse make.

Closes #168

cockpit-project/cockpituous,cockpit-project/cockpituous