configs: ls1021atwr: Fix distro_bootcmd for QSPI boot

Due to a typo, "run qspi_bootcmd" and "env exists secureboot" got
concatenated instead of being separated by a semicolon.

Signed-off-by: Vladimir Oltean <olteanv@gmail.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>