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>