This stuff how do you actually do it from the u-boot console?
NOR recovery
Once you have U-Boot running, you can get various images for example via tftp and write it to nor.
To download image from tftp server you need to get your network started, decide where to load it and load it. Assuming you have tftp server running on computer with ip 192.168.1.1 on WAN network, following U-Boot commands will get you image into RAM.
setenv autoload no
dhcp
setenv serverip 192.168.1.1
tftpboot ${kernel_addr_r} image
Now you have a file originally called image in RAM on your router. To write to NOR you have to know where it belongs.
To reflash U-Boot, your image file on tftp will be uboot-turris-omnia-spl.kwb and you need to write it from the beginning of the NOR.
sf probe
sf update ${kernel_addr_r} 0 ${filesize}
To recover rescue system, your image file will be omnia-initramfs-zimage and you need to start writing it after first megabyte that is reserved for U-Boot
sf probe
sf update ${kernel_addr_r} 0x00100000 ${filesize}
After that, you can call reset command or hit a reset button and see your router booting again.