Hello,
You can add a sequence to the init.rc file as below:
First, create a script with the command that you want to use. In this case, will be:
am broadcast -a android.intent.action.MEDIA_MOUNTED -d file:///mnt/sdcard
Then add in the init.rc file the below commands:
service script /system/bin/script.sh
user root
oneshot
Remember to store the script file in the path that you are specifying on the commands of the init.rc
Unfortunately, we are unable to assist you more since the board that you are using is from toradex. For more help I recommend you to go with Toradex. I apologize for the inconveniences this could give you.
Best regards,
Diego.