mb/emulation/qemu-riscv: Change to -bios option

This changes the virt target so that it can be run with the -bios option
and a pflash backend for the flash. QEMU can now be run as follows:

qemu -M virt -m 1G -nographic -bios build/coreboot.rom \
        -drive if=pflash,file=./build/coreboot.rom,format=raw

coreboot will start in DRAM, but still have a flash to put CBFS onto and
to load subsequent stages and payload from.

Tested bootflow:
coreboot -> OpenSBI -> Linux -> u-root

Signed-off-by: Maximilian Brune <maximilian.brune@9elements.com>
Change-Id: I009d97fa3e13068b91c604e987e50a65e525407d
Reviewed-on: https://review.coreboot.org/c/coreboot/+/80746
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: ron minnich <rminnich@gmail.com>
Reviewed-by: Philipp Hug <philipp@hug.cx>

Documentation/mainboard/emulation/qemu-riscv.md

@@ -3,6 +3,9 @@
 ## Building coreboot and running it in QEMU
 
 - Configure coreboot and run `make` as usual
-- Run `util/riscv/make-spike-elf.sh build/coreboot.rom build/coreboot.elf` to
-  convert coreboot to an ELF that QEMU can load
-- Run `qemu-system-riscv64 -M virt -m 1024M -nographic -kernel build/coreboot.elf`
+
+Run QEMU
+```
+qemu-system-riscv64 -M virt -m 1G -nographic -bios build/coreboot.rom \
+                    -drive if=pflash,file=./build/coreboot.rom,format=raw
+```