imx-vpu: add VPU firmware selection

As the imx-vpu package needs the firmware to be loaded into the VPU it
makes sense to have the latter package selected by default.

Signed-off-by: Gary Bisson <bisson.gary@gmail.com>
Reviewed-by: Luca Ceresoli <luca@lucaceresoli.net>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>