Symlink fixes

See merge request !1138

Angelos Evripiotis authored Feb 05, 2019 and Jürg Billeter committed Feb 11, 2019

Replace the instructions for pyflame+flamegraph with simpler ones for
snakeviz. For our general use-case this seems to be easier and better.
Usage of this tool was demonstrated at the 2019 BuildStream Gathering in
January by Daniel Silverstone, when presenting the aggregate results of
profiling on many target environments.

Here is the relevant mailing list thread:

"Profiling before the gathering"
https://mail.gnome.org/archives/buildstream-list/2019-January/msg00057.html

contributing: snakeviz replaces pyflame+flamegraph

See merge request !1129

_context.py: Add cache_buildtrees global user context, the default
of which is set to by default to True via the addition of
cache-buildtrees to userconfig.yaml cache group.

app.py & cli.py: Add --cache-buildtrees/--no-cache-buildtrees as
a bst main option, which when passed can override the default or
user defined context for cache_buildtrees.

tests/completions/completions.py: Update for the added flag.

Artifacts can cached explicitly with an empty `build tree` when
built via the cli main options or user config. Default behaviour
is to still create and cache expected buildtrees.

element.py: _cache_artifact() Check if context for cache_buildtrees
has been set to False, if so skip attempting to export the
build-root. Element types without a build-root are cached with an
empty buildtree regardless. Update _stage_sources_at() to warn the
user that the buildtree import is empty.

tests/integration: Add test to artifact.py for the optional caching
of buildtree content from bst build. Rename build-tree.py to
shellbuildtrees.py to reflect included test cases, add test for
empty buildtree warning.

NEWS: Add entry for new option.