By "bootable Linux", you presumably mean something that could live-boot a PC? No. The Humax central processor is not x86 based, and I doubt booting from the USB is built into the hardware. If you want a graphical desktop you would need to work out how to drive the graphics hardware and provide drivers. Further, only a very light build of Linux would have enough resources to run.
In short, you might as well use what's there already, and settle for the command line.
You really need a Firmware developer to answer this (I'm not one either), I guess you are referring to This Kind Of device, Rather than something that would boot from a P.C. USB stick as Centos is Linux
Thanks guys - pretty much why I addressed the subject line as I did.
I'm familiar with cross compilation etc. also my interest is in bootability rather than features such as GUI's
and large footprints. I mentioned centos merely as an example to the idea across.
If anyone has any further info on bootability of the hardware I'd welcome it.
I think it would take a special version of the firmware boot process to go and look for a bootable image on the USB.
However, taking one step back from a literal "boot from USB", we do already have a couple of special "boots": one to go into Maintenance Mode and another to start HDR Mode on a HD-FOX. Control is passed to a different image from the command line (or a script). You could start up a different executable in the same way.