mx7ulp_evk: Move CONFIG_CMD_BOOTZ to Kconfig

CONFIG_CMD_BOOTZ symbol does not work in board config file
anymore, so fix this by moving it to Kconfig.

Signed-off-by: Fabio Estevam <fabio.estevam@nxp.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>