feat(hack): add scripts to drive the dev environment headless

Start QEMU in the background with a QMP socket and console logs, take
screenshots, press keys and wait for serial output, so the graphical boot
menu can be driven without a terminal or a window. boot.sh gets opt-in
knobs for this and behaves as before otherwise.

hack/dev/run/start.sh

@@ -0,0 +1,39 @@
+#!/bin/sh
+# Starts the dev environment in the background with no window, a QMP socket and serial logs.
+# Usage: start.sh [arch]
+# Environment: SKIP_BUILD=1 reuses target/final, SPROUT_CONFIG_NAME picks the config (default
+# graphical), START_TIMEOUT is the seconds to wait for QEMU (default 900 to allow for a build).
+set -e
+
+cd "$(dirname "${0}")/../../.." || exit 1
+
+[ -n "${1}" ] && TARGET_ARCH="${1}"
+. "hack/dev/run/lib.sh"
+
+is_running && die "a ${TARGET_ARCH} QEMU is already running, stop it with hack/dev/run/stop.sh"
+
+rm -rf "${RUN_DIR}"
+mkdir -p "${SHOT_DIR}"
+rm -f "${QMP_SOCK}"
+
+export SPROUT_CONFIG_NAME="${SPROUT_CONFIG_NAME:-graphical}"
+export QEMU_HEADLESS=1
+export QEMU_QMP="${QMP_SOCK}"
+export QEMU_SERIAL_FILE="${SERIAL_LOG}"
+export QEMU_CONSOLE_FILE="${CONSOLE_LOG}"
+
+nohup ./hack/dev/boot.sh "${TARGET_ARCH}" </dev/null >"${QEMU_LOG}" 2>&1 &
+echo "${!}" >"${PID_FILE}"
+
+# boot.sh builds first and then execs QEMU, so the pid stays the same.
+DEADLINE=$(($(date +%s) + ${START_TIMEOUT:-900}))
+while [ ! -S "${QMP_SOCK}" ]; do
+	if ! is_running; then
+		tail -n 20 "${QEMU_LOG}" >/dev/stderr
+		die "QEMU exited before it was ready, see ${QEMU_LOG}"
+	fi
+	[ "$(date +%s)" -ge "${DEADLINE}" ] && die "timed out waiting for QEMU, see ${QEMU_LOG}"
+	sleep 1
+done
+
+echo "${RUN_DIR}"