toradex: allow custom fdt board setup in board file

The config block support currently uses the ft_board_setup function
to patch the device tree with config block information. However, this
does not allow to patch the device tree with board specific information.
Rename the common setup function to ft_common_board_setup and use the
call it from the board files directly.

Signed-off-by: Stefan Agner <stefan.agner@toradex.com>