how did you produce your rooted image in the first place, please?
On the Hudl 1, iirr a manufacturer-specific tool was used to copy the /system partition byte by byte, then that image mounted on a Linux system as an ext* filesytem, the su binary copied in, and finally the new image distributed to be written back with the same tool. Are you doing roughly the same here, and is it ADB, or flashtools, or what, that is taking the place of the RK specific tool?
If not, then please post a summary of the process you used, please.
Please do let me know how it was done, as I am keen to know how to repeat the entire process, rather than simply starting from a file posted by an online guru.
I've written a script to automate what Paul did, so that the same script will continue to work until the partition offset and length changes. Original motivation was that neither of the posted roms were compatible with my brand new Hudl.
My script also crashes out gracefully when the partition offest is wrong, minimising the risk of bricking.
It also keeps a copy of the partition before and after the rooting, to allow you to revert to how it was before (maybe if the partitions moved - the data goes back exactly where it came from so this gives the best chance of recovering). The copy of the rooted system will allow you to roll-back at a later date to the point just after rooting.
This has been tested on the latest version of the hudl software, JDQ39.20140424.153851 and works a treat.
It works either before or after the user has turned the hudl on for the first time and filled in all the various info, so you can produce a factory-fresh rooted machine, or just root it in its current state.
Two versions, the lite version installs the su from clockworkmod and leaves you to install your favoured superuser app. In this case the app will not be a system app (unless it has its own internal install method, like for example supersu)
I've chosen the clockworkmod implementation as it is open source, and in my view it is particularly valuable to have open source software when rooting a device, as you don't otherwise know what else the app is doing.
The full version installs the full clockwordmod superuser app as a system app (in the hope that it will thus be more resistant to being uprooted). It does make the download bigger, but neither is anywhere near the size of the full system rom
to use rkflashtools on the pi you will need to compile them yourself (*) The pi is a 32bit ARMv6 machine, and the supplied tools are for 64bit Intel or AMD. The one flurry is offering will be 32 bit but again for Intel/AMD x86 not for any version of ARM.
And you need to conect directly to the host, not through a hub, so if your USB sockets are alreadu in use, like for the keyboard or something, then even with the correct binary it will not work.
(*) Or persuade the raspian maintainers, or any of the oher pi distros, to add them as a supported package.You may just be the first person on the planet to ask that question....