I want to know what I exactly need to do for booting from the sdcard. According to the document at https://github.com/96boards/documentation/wiki/HiKeyGettingStarted, in the section ‘3. Using a SD Card’, it mentioned that the ‘fastboot flash boot boot-fat.img’ command is needed for booting from the sdcard. This is confusing to me, because the sdcard should have the boot partition with boot image installed already, why flashing is still needed here. Furthermore, what is the target of this flashing operation? emmc or sdcard?