To change Redboot configuration from shell, use fconfig tool:

(this example shows how to change configuration to load OpenWRT kernel from mtd partition kern1)

1) backup current redboot config to file:

dd if=/dev/mtd3 of=mtd31-fsg.bin

2) change the configuration:

fconfig -w -d mtd31-fsg.bin -n boot_script_data -x 'fis load kern1\exec -c "console=ttyS0,115200 root=/dev/sda1 init=/etc/preinit"\'

3) erase current configuration:

erase /dev/mtd3 0 1 1

4) write the new configuration:

dd if=mtd31-fsg.bin of=/dev/mtd3