freescale-imx: change platform choice to be common

That way all the i.MX packages can inherit from that information,
for example, the soon-to-be-introduced imx-vpu package.

Signed-off-by: Gary Bisson <bisson.gary@gmail.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
[yann.morin.1998@free.fr: the part about gpu-viv-bin-mx6q moved to
 its own patch; rework the commit log]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Hadrien Boutteville <hadrien.boutteville@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>