board: ti: j721s2: Set fdtfile from C code instead of findfdt script

We now can provide a map and have the standard fdtfile variable set from
code itself. This allows for bootstd to "just work".

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

Reviewed-by: Jonathan Humphreys <j-humphreys@ti.com>
Reviewed-by: Roger Quadros <rogerq@kernel.org>
Signed-off-by: Nishanth Menon <nm@ti.com>