Booting from SD with U-boot

Could someone please tell me which command do I need to boot just one time from the SD card in order to erase my internal memory?

On which device? Different ARM SoCs have different boot orders.

Never mind I just typed boot and it functioned the second time. The problem is that I wanted to erase my internal memory and I did it with shred… I was busy doing something else after one hour of just staring at the screen and preventing the machine go into sleep mode because I knew it it has problems in this concern. I guess it was just 10% away from finishing the whole thing and it went into sleep and hasn’t finished the procedure. Now I wiped the SD card and flashed the image again and it doesn’t even boot from the sd…

Do you have any idea if erasing the SD from Mac with eraseDisk FAT32 is relevant? Or do I have to use a specific format? Or as long as you finish it with etcher it doesn’t matter?