Add a --bootcmd option to the build-qemu script that allows passing a boot command to QEMU via fw_cfg. The bootcmd is written to the "opt/u-boot/bootcmd" fw_cfg entry, which can be read by U-Boot's EVT_BOOTCMD handler. Co-developed-by: Claude <noreply@anthropic.com> Signed-off-by: Simon Glass <sjg@chromium.org>