Comment # 1 on bug 994415 from Bernhard Wiedemann

incidentally, I got a SLES-12-SP1 system with
console=tty console=ttyS1,115200
and was seeing those boot messages on serial

Can you paste your full
cat /proc/cmdline

could you try
# cat /proc/consoles 
ttyS1                -W- (EC p a)    4:65
tty0                 -WU (E  p  )    4:1