At the moment the regular meta-raspberrypi SD card class does not allow
for optional inclusion of extra files in the SD card boot partition
without requiring those same files be present in the root filesystem.
This affects hypervisor binaries, such as Xen, which do not necessarily
belong within the guest filesystem.

As a workaround, inherit the sdcard_image-rpi.bbclass and clone the
main IMAGE_CMD function, adding the Xen-specific deploy that is needed.
This can be retired when the parent class has extensibility added.

Signed-off-by: Christopher Clark <christopher.w.clark@gmail.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>