board: ten64: opt out of fsl_setenv_bootcmd

Our bootcmd is the same regardless of where the SoC
loaded it's code from, so we don't want
fsl_setenv_bootcmd to do anything.

Signed-off-by: Mathew McBride <matt@traverse.com.au>
Reviewed-by: Peng Fan <peng.fan@nxp.com>