manual: faq: add workaround to avoid rebuilding the buildroot toolchain after
each clean
manual: minor fix in patch-policy.txt
manual: misc. post-{build,image} scripts fixes

* update post-build scripts mechanism that now support several scripts
* fix formatting
manual: update {deprecated,package}-list.txt
Makefile: add to the release target a warning about the manual updates.

The manual-update-lists target is not added to the dependencies of the
release target to allow editing the generated files.