Rewrite the Package builds section

This patch rewrites the Package builds section adding more details about
package builds using the SDK, devroots and snapshots of APT archives.

Signed-off-by: Peter Senna Tschudin <peter.senna@collabora.com>

content/architecture/long-term-reproducibility.md

@@ -308,14 +308,55 @@ the remaining planned improvements.
 
 ## Package builds
 
-Package builds happen on OBS which does not have snapshotting capabilities and
-always builds every package on a clean, isolated environment built using the
-latest package versions for each channel.
-
-Since the purposes taken in account in this document do not involve large scale
-package rebuilds, it is recommended to use the SDK images and the devroots in
-combination with APT archives snapshots to rebuild packages in an
-environment closely matching a past build.
+Apertis uses [OBS](https://openbuildservice.org/) for building packages,
+however since the purpose of the long term reproducibility does not involve
+large scale package rebuilds, we recommend using the [Apertis
+SDK]({{< ref "software-development-kit.md" >}}) for building packages. In addition to
+the SDK a [devroot]({{< ref "sysroots-and-devroots/#devroot" >}}) and a snapshot
+of the APT repositories are also required.
+
+We recommend the SDK release to match the release of Apertis in the target
+device, and it is important that both the release and architecture of the
+devroot and of the snapshot of the APT repositories matches the target device.
+As an example, if the target is an `ARM64` device, the devroot and the snapshot
+of the APT repositories should also be for `ARM64`.
+
+See the [devroot]({{< ref "sysroots-and-devroots/#devroot" >}}) for more
+information.
+
+For the long term reproducibility we recommend the product team to have a
+backup copy of:
+ * The Apertis SDK: Matching the Apertis release in use by the target device.
+ * The Apertis devroot: Matching the Apertis release and the architecture of
+   the target device.
+
+At the time of writing OBS supports native and emulated builds, and we have
+successfully deployed emulated builds on the Bosch Developer Cloud. While
+emulated builds are expected to work in most cases, they may require
+changes to the packages being build using emulation, and they may produce
+different results when compared to native builds.
+
+For more information about emulated and native builds see
+[Build infrastructure on Intel x86-64]({{< ref "x86-build-infrastructure.md" >}}).
+
+Building packages on the SDK uses emulation, and while this approach is
+currently expected to be satisfactory, it may produce different results when
+compared to native builds. In the unlikely event of emulated builds not being
+satisfactory, we describe a mitigation on the section
+[Future work on package builds]({{< ref "long-term-reproducibility.md#future-work-on-package-builds" >}})
+
+### Future work on package builds
+
+We recommend using the Apertis SDK for building packages in the context of the
+long term reproducibility to enable product teams to build packages without
+depending on custom OBS deployments.
+
+However using OBS for building packages in the context of the long term
+reproducibility may offer advantages, when large package rebuilds are needed or
+when native builds are required.
+
+We will consider for future work automating the OBS configuration to support
+building packages for the long term reproducibility.
 
 # Recommendations for product teams