To build a bootable SD card

Step 1: Build spl-mmc binary (there is a compiled binary in repo, can skip build and use our version as-is)

$ make CROSS_COMPILE=riscv64-unknown-elf- p=sun20iw1p1 mmc

Step 2: Format SD card with FAT 32 filesystem.

Step 3: Write mmc image to boot sector of SD card. We wrote a quick script to do this (but script works only on macOS).

$ jz_write_bootsector.sh

See README-sdcard.txt for rest of instructions about using the SD card for boot.