board: ti: am62ax: Set fdtfile from C code instead of findfdt script

Stop using the findfdt script and switch to setting the fdtfile from
C code.

While at this, replace findfdt in environment with a warning as it is
no longer needed

Signed-off-by: Nishanth Menon <nm@ti.com>
Reviewed-by: Jonathan Humphreys <j-humphreys@ti.com>