ARM: imx: cm_fx6: env: try to determine dtb to use

ARM: imx: cm_fx6: env: try to determine dtb to use

Some distributions like Fedora expect U-Boot to select a proper
devicetree. Since there are several variants of the cm-fx6 module
featuring different SoC variants and the module can be paired with
several baseboards, it is not viable to hardcode a filename.

Instead, follow the lead of other i.MX6 based devices and try to
determine the devicetree to use with the help of the board name
and the SoC variant exported by the board code, before calling the
distro bootcommand.

For now, only for the Utilite Pro a proper devicetree filename is
known but further variants of the Utilite Computer or other devices
based on the cm-fx6 module may be added in the future.

Signed-off-by: Christopher Spinrath <christopher.spinrath@rwth-aachen.de>