imx6: rename target to 'imx'

This is first step in migrating to a generic i.MX target which in the
next steps will also get divided into arch-specific subtargets.

In the result, this will make it possible to support, within a single
target, also other modern NXP i.MX families, like the i.MX 7, i.MX 8
or recently introduced i.MX 9.

Signed-off-by: Piotr Dymacz <pepe2k@gmail.com>