buildroot: don't print service logs to the serial console

[a-nogikh]

They don't really contribute to making kernel bugs easier to debug, but tend to litter the logs, causing curious output interleavings and report parsing failures.

E.g. in case of this bug: https://syzkaller.appspot.com/bug?extid=524a32a528b99d65b7fb

Oct 21 06:58:24 syzkaller daemon.err dhcpcd[5056]: libudev: received NULL device
Oct 21 06:58:24 syzkaller daemon.err dhcpcd[5056]: libudev: received NULOct 21 06:58:24 syzkaller daemon.err dhcpcd[5056]: libudev: received NULL device
Oct 21 06:58:24 syzkaller daemon.err dhcpcd[5056]: libudev: received NULL device
Oct 21 06:58:24 syzkaller daemon.err dhcpcd[5056]: libudev: received NULL device
Oct 21 06:58:24 syzkaller daemon.err dhcpcd[5056]: libudev: received NULL device
Oct 21 06:58:24 syzkaller daemon.err dhcpcd[5056]: libudev: received NULOct 21 06:58:24 [   69.594319][    C3] ------------[ cut here ]------------
syzkaller daemon[   69.595934][    C3] WARNING: CPU: 3 PID: 1196 at net/mac80211/tx.c:5038 __ieee80211_beacon_get+0x14ac/0x16b0
.err dhcpcd[5056[   69.598652][    C3] Modules linked in:
[   69.600341][    C3] CPU: 3 UID: 0 PID: 1196 Comm: kworker/u32:11 Not tainted 6.12.0-rc3-syzkaller-00454-gdb87114dcf13 #0

Another action item here is to check whether we could still have adequately parsed the crash log.

[melver]

It might be enough to just pass systemd.log_level=crit or systemd.log_level=err on kernel command line.

[a-nogikh]

It's quite likely that our Buildroot images don't have systemd at all

[melver]

It's quite likely that our Buildroot images don't have systemd at all

Good point. I thought some of the images are still Debian based.