TFTP, the protocol, doesn't place many requirements on the OS environment. Any Linux distribution that includes sockets and memory management will suffice. That's pretty much ALL OF THEM.
I would suggest choosing the distribution based on something else. (memory/flash footprint size, other SW you want to run, clever iconic logo, etc)
As a "boot image server", the plug just needs access to enough static storage (via Ethernet, USB or SDIO) to hold the images it has to serve.
I used an OpenRD-like platform (same Marvell processor) as a "DHCP/BootP/boot image server" for a group of DSPs in a previous project and it worked out well. This was "homebrew" server software so I'm useless for off-the-shelf SW recommendations. Sorry.