forked from Mirrors/opensbi

U-Boot v2020.07 release adds SPL support to SiFive HiFive Unleashed. Update the doc to mention that detailed build instructions are in the U-Boot doc. This also adds detailed command line description to show how to boot U-Boot v2020.07 S-mode payload in the QEMU chapter. Signed-off-by: Bin Meng <bin.meng@windriver.com> Reviewed-by: Atish Patra <atish.patra@wdc.com>