Introduce dev distro feature

Simplify Cassini configurarion by introducing 'dev.yml' to replace use of 'cassini-dev.yml' image.

The following are updated

• dev.yml added to the kas menu and documentation
• cassini-dev.yml is removed from kas menu and documentation
• cassini-dev builds are replaced with cassini/dev
• cassini-dev rules are replaced by cassini + dev rules

Changelog: feature Signed-off-by: Adam Johnston adam.johnston@linaro.org