tao3530: Fix usage of mmc rescan

Currently "bootcmd" does not work as intended but instead prints MMC usage
information and goes directly to "nandboot".

Follow what the commit 66968110 ("configs: Fix usage of mmc rescan")
does for other boards prior to support for TechNexion TAO3530 SoM was
added.

Signed-off-by: Jarkko Nikula <jarkko.nikula@bitmer.com>