Console baud rate - nodesign/openwrt-ava GitHub Wiki
Uboot uses speed of 115200bps for console output but default openwrt configuration switch it to 57600bps.
To change this parameter you have to go to
vi ./target/linux/ramips/dts/mt628an.dtsi
and then change
bootargs = "console=ttyS0,115200 ";
It seems that this is possible by only editing menuconfig but I didn't find a way so far. Here is a partial tutorial in that purpose https://wiki.openwrt.org/doc/recipes/serialbaudratespeed#second.method
Change for baud rate has been changed in the source and committed