goosnarrggh

Nice. Thank you.

This is amazing, thank you goosnarrggh!
 
The dtbo-file works for me exactly as described. My armbian is booting again and you saved me from completly giving up on the bananapi.
 
If it helps at all to narrow down where the issue comes from: The current (and some earlier) minimal image of armbian worked for me initally, but after installing and updating some packages (including a rebuild of initramfs), I consistently ran into the freeze at "Scanning for btrfs filesystems".

Update: Yes, a dtbo will work to reduce the bus from 8-bits to 1-bit and make the board bootable.
 
This overlay does the trick for me:
sun8i-r40-bananapi-m2-ultra-1bit-emmc.dtssun8i-r40-bananapi-m2-ultra-1bit-emmc.dtbo
 
To test this for yourself: (You will need a working Linux machine to mount the SD card and make these changes.)
Burn a copy of the current rolling release image onto a microSD card. Copy the dtbo file into the /boot/overlay-user/ subdirectory of the card. (You will need to use root/superuser access to have permission to modify the content of this directory on the SD card. Create that subdirectory if it doesn't already exist.) Edit the file /boot/armbianEnv.txt. (You will need to have root/superuser access to have permission to edit this file on the SD card.) If there's no line starting with "user_overlays=" in the file, then just add a new line to the end of the file: user_overlays= sun8i-r40-bananapi-m2-ultra-1bit-emmc Otherwise, if a line starting with "user_overlays=" is already present in the, then add a space to the end of that line's existing contents, and add sun8i-r40-bananapi-m2-ultra-1bit-emmc to the end of it.  
Note: I linked both the dtbo binary, and the dts source code which created it. Only the dtbo file is needed for a quick test; if you wish you could compile the dts file from source code and prove that my work is reproducible.

Update: Yes, a dtbo will work to reduce the bus from 8-bits to 1-bit and make the board bootable.
 
This overlay does the trick for me:
sun8i-r40-bananapi-m2-ultra-1bit-emmc.dtssun8i-r40-bananapi-m2-ultra-1bit-emmc.dtbo
 
To test this for yourself: (You will need a working Linux machine to mount the SD card and make these changes.)
Burn a copy of the current rolling release image onto a microSD card. Copy the dtbo file into the /boot/overlay-user/ subdirectory of the card. (You will need to use root/superuser access to have permission to modify the content of this directory on the SD card. Create that subdirectory if it doesn't already exist.) Edit the file /boot/armbianEnv.txt. (You will need to have root/superuser access to have permission to edit this file on the SD card.) If there's no line starting with "user_overlays=" in the file, then just add a new line to the end of the file: user_overlays= sun8i-r40-bananapi-m2-ultra-1bit-emmc Otherwise, if a line starting with "user_overlays=" is already present in the, then add a space to the end of that line's existing contents, and add sun8i-r40-bananapi-m2-ultra-1bit-emmc to the end of it.  
Note: I linked both the dtbo binary, and the dts source code which created it. Only the dtbo file is needed for a quick test; if you wish you could compile the dts file from source code and prove that my work is reproducible.

Thank-you -- I can confirm this fixes the issue and allows the board to finally boot in something other than their provided kernel version 3.3.

I tried quick test with dtbo file you provided and it works. This is the first time I have this board up and running. It would be great if you changes could somehow be included in future images provided for download.

Update: Yes, a dtbo will work to reduce the bus from 8-bits to 1-bit and make the board bootable.
 
This overlay does the trick for me:
sun8i-r40-bananapi-m2-ultra-1bit-emmc.dtssun8i-r40-bananapi-m2-ultra-1bit-emmc.dtbo
 
To test this for yourself: (You will need a working Linux machine to mount the SD card and make these changes.)
Burn a copy of the current rolling release image onto a microSD card. Copy the dtbo file into the /boot/overlay-user/ subdirectory of the card. (You will need to use root/superuser access to have permission to modify the content of this directory on the SD card. Create that subdirectory if it doesn't already exist.) Edit the file /boot/armbianEnv.txt. (You will need to have root/superuser access to have permission to edit this file on the SD card.) If there's no line starting with "user_overlays=" in the file, then just add a new line to the end of the file: user_overlays= sun8i-r40-bananapi-m2-ultra-1bit-emmc Otherwise, if a line starting with "user_overlays=" is already present in the, then add a space to the end of that line's existing contents, and add sun8i-r40-bananapi-m2-ultra-1bit-emmc to the end of it.  
Note: I linked both the dtbo binary, and the dts source code which created it. Only the dtbo file is needed for a quick test; if you wish you could compile the dts file from source code and prove that my work is reproducible.