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

[hnez]

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.

This PR is based on a discussion in PR https://github.com/linux-automation/meta-lxatac/pull/125#discussion_r1547540978. There @a3f also brings up other points that concern the flashing guide in the README:

AFAIK, the watchdog is only enabled before booting Linux (boot command), so wd -x directly after DFU bring-up doesn't matter for watchdogs. As for stopping the automatic boot process, fastboot flash has the same effect, see: https://github.com/barebox/barebox/blob/5423885658da4beddec619c1ec4b1cb741c68554/common/fastboot.c#L814

If this change fixes something, I suspect the issue is elsewhere.

TODO before merging:

• [ ] Inverstigate what @a3f said about the effect of wd -x and change the README accordingly. The investigation is a separate topic from the change introduced here.

[hnez]

This PR got kind of forgotten some time ago. I think the change is good, so we should get this merged.