toradex: Use checkboard() instead of show_board_info()

Boards can use a sysinfo driver if a particular model name is needed.
Update this board to use checkboard() directly, rather than having a
weak function laid on top of a weak function.

Make all the checkboard() functions call the new tdx_checkboard() so
that the same information is displayed.

Signed-off-by: Simon Glass <sjg@chromium.org>