scripts: list images and packagegroups from fsl layers

Generate release-notes/source/images.inc and
release-notes/source/packagegroups.inc based on recipes in
meta-fsl-arm, meta-fsl-arm-extra and meta-fsl-demos.

The "heuristic" to determine what is an image/packagegroup is pretty
lame: it just checks if the recipe name contains `image' or
`packagegroup'.

The generated files contain an rst list mapping the image and
packagegroup recipe names to their corresponding descriptions.

The generated files (.inc) have intentionally not been added to this
commit.

Signed-off-by: Mario Domenech Goulart <mario@ossystems.com.br>
scripts: bitbake-metadata2doc.sh: use a fresh build directory

Before this change we were using a hardcoded build directory
("build").  That approach could lead to problems if the build
directory already existed and contained settings other than the
default ones (setup-environment would not overwrite conf/local.conf
and/or bblayers.conf).

This patch changes that by creating a fresh build directory, thus
making setup-environment create new configuration files with the
default settings, which will make scripts generate more deterministic
results.

Signed-off-by: Mario Domenech Goulart <mario@ossystems.com.br>
README: add "Contributing" section

Adapted from meta-fsl-arm README's "Contributing" section.

Signed-off-by: Mario Domenech Goulart <mario@ossystems.com.br>