meta-lxatac-bsp: barebox: increase boot timeout when serial booting

When someone loads barebox via serial download instead of e.g. from the
eMMC it is very likely that they want to upload an image via fastboot
next and do not want to autoboot into linux right away.

Give them a bit more time to interrupt the boot process.

Signed-off-by: Leonard Göhrs <l.goehrs@pengutronix.de>

meta-lxatac-bsp/recipes-bsp/barebox/files/lxatac/env/init/20_serial_timeout.sh

@@ -0,0 +1,5 @@
+#!/bin/sh
+
+if [ "${bootsource}" = serial ]; then
+    global.autoboot_timeout=30
+fi