Adapt genmsg to handle msg files relatively to the current
cmake path. Without this modification genmsg failed to generate
messages because it pass a path without sysroot to the python
code.

Signed-off-by: Stefan Herbrechtsmeier <stefan@herbrechtsmeier.net>

Signed-off-by: Stefan Herbrechtsmeier <stefan@herbrechtsmeier.net>

adding the genmsg recipe, which is the first recipe that also creates some python modules and hence requires some more ingredients in the recipe