As far as the boot process, Openwrt (linino) goes through a few steps while booting up. The console is attached to the serial port to allow interaction in case bad things happen so you can fix them, without this you could end up "bricked" with no way to "unbrick".
Specifically first there is an opportunity to stop uboot so you can flash/boot if the image is bad. Next is failsafe mode which allows you to fix things that are messed up in the linux system configuration, next is the console started in inittab which allows you access if you lose network connectivity. While you could run for years without needing these things they are nice to have, kinda like airbags in a car, if you need them you are VERY glad you had them.
I believe the only way to disable the first two early stages would be to compile an image after appropriate changes have been made to uboot and kernel parameters.