[packagegroup-gumstix] Add gumstix packagegroup

The Gumstix package repository at [1] should contain packages that are
generally useful to Gumstix users but may not be necessarily included
in standard images.  This packagegroup provides a target for any packages
we want to build for and include in the package repository.

[1] https://packages.gumstix.com/

Signed-off-by: Ash Charles <ashcharles@gmail.com>
Merge branch 'next' of git://github.com/gumstix/meta-gumstix-extras into next