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/wait-serial.sh

@@ -0,0 +1,29 @@
+#!/bin/sh
+# Waits for a regex to show up on the firmware or Linux console. Exits 1 on timeout or if QEMU exits.
+# Usage: wait-serial.sh <regex> [timeout-seconds]
+set -e
+
+cd "$(dirname "${0}")/../../.." || exit 1
+
+. "hack/dev/run/lib.sh"
+
+[ -n "${1}" ] || die "usage: wait-serial.sh <regex> [timeout-seconds]"
+
+DEADLINE=$(($(date +%s) + ${2:-60}))
+while :; do
+	if cat "${SERIAL_LOG}" "${CONSOLE_LOG}" 2>/dev/null | grep -aEq -- "${1}"; then
+		exit 0
+	fi
+	if ! is_running; then
+		echo "QEMU exited while waiting for: ${1}" >/dev/stderr
+		break
+	fi
+	if [ "$(date +%s)" -ge "${DEADLINE}" ]; then
+		echo "timed out waiting for: ${1}" >/dev/stderr
+		break
+	fi
+	sleep 0.5
+done
+
+tail -n 15 "${SERIAL_LOG}" "${CONSOLE_LOG}" 2>/dev/null >/dev/stderr || true
+exit 1