Comment 21 for bug 638384

Here's an hopefully not too complex proposal which solves all issues I'm aware of:
* add the GPG key of the current Offspring instance to linaro-image-tools
* change linaro-media-create to verify rootfs.asc and hwpack.asc when passed --binary rootfs --hwpack hwpack args
* change linaro-hwpack-create to include PPA keys when the hwpack includes PPAs and linaro-hwpack-install to install them and setup the corresponding PPAs when the hwpack includes keys and PPAs
* provide a tool which downloads and verifies Linaro images (for instance, shell script or pygtk UI)