I was using 1.1.8 with the serial console device speed set to 115200 and it worked fine. After upgrading to 1.2.0-rc5 and applying the existing config through the 'add system image' process 115200 only worked through grub, but after that it used 9600. The correct config line was in the config "set system console device ttyS0 speed '115200'". However, I had to set the speed to 9600 and commit and then set it to 115200 and commit to get it to work with 115200 all the way to login and beyond. I saved and rebooted between each change so I'm not sure if that was required, but I imagine that the commit action cleaned it up.
set system console device ttyS0 speed '9600'
set system console device ttyS0 speed '115200'