I am wondering how to us this file which is the output of Buildroot.
Can I dd it straight over /
Do I need to turn it into a jffs2 .img somehow?
That's an FS image so you can in principle dd it, so long as in the process you don't trash the running fs!
For all the ways to stick this in your flash, have a look at https://www.avrfreaks.net/wiki/in.... If you're running a recent buildroot (and IIRC you are) then you'll also have a -usr image which has to be stuck in dataflash. Once again the instructions are on the aforelinked page :)
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