I was relying myself on document "LS1012ARDBGSG.pdf", rev5 8/2018, a following especially the chapter "§14 Deploying Ubuntu userland on LS1012ARDB".
My aim is to be able to fully build firmware (that seems to works) and the boot partition + RFS (which must be put on USB stick in my case indeed).
To answer your question: yes I prepped the stick following instructions given in "§14.5.2 Option 2: By preparing SDHC card on the board". I did it the target way via udhcpc method. So yes I used flex-installer as explained expect my device was not /dev/mmcblk0 but /dev/sda. At the end I got also four partitions and enterer 3rd partition (think labelled "backup") where I put both tarballs coming out of a "flex-builder -m ls1012ardb" just like the firmware above. No special settings yet, just following instructions and using a USB stick instead of an SD card yet, that's it.
What I am doing now is I completely cleaned my builds (inlc. rfs) and redo it clean way then reflash and I'll see.
I've you've other elements please share so I don't loose more time on looking for solution (even if I like doing that :smileywink:).
Thx Christophe