am335x_evm: Consolidate DFU environment parts into the DFU part of the file

To make managing the environment easier, add DFUARGS to
CONFIG_EXTRA_ENV_SETTINGS.  Then we set DFUARGS down in the DFU part of
the file, and include (or not) the NAND part, based on if NAND is set.

Signed-off-by: Tom Rini <trini@ti.com>