board: tegra: convert boards to text env

Convert boards to use text based env. This is the first stage of
conversion, common inclusions should be converted next.

Acked-by: Francesco Dolcini <francesco.dolcini@toradex.com> # Toradex Apalis TK1
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Svyatoslav Ryhel <clamor95@gmail.com>