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>
RN: Add project_name as global variable

Signed-off-by: Daiane Angolini <daiane.angolini@freescale.com>
RN: Add section for PACKAGEGROUPS and images

The files packagegroups.inc and images.inc are not included
on purpose. Those files are automatically generated by the
updated script.

Signed-off-by: Daiane Angolini <daiane.angolini@freescale.com>
RN: Fix title underline length

This avoid a warning

   WARNING: Title underline too short.

Signed-off-by: Daiane Angolini <daiane.angolini@freescale.com>
script: Add gstreamer1.0

This user space package is important to be tracked.

Signed-off-by: Daiane Angolini <daiane.angolini@freescale.com>
RN: Updates from bitbake-metadata2doc.sh

Signed-off-by: Daiane Angolini <daiane.angolini@freescale.com>
RN: Updates from script only on ack-sourcers file

Signed-off-by: Daiane Angolini <daiane.angolini@freescale.com>