tomu tiny EFM32 that fits in a USB port at lca 2018. More information on Tomu.
Mine came with the serial bootloader (it presents a USB CDC ACM device). I didn't manage to get to the update booth to reflash it at LCA.
I had some difficulty getting the new bootloader on it when I pulled out recently to try to play with it, but since I got it working, here's a short runthrough of how I got it updated.
I use Fedora 28 but this info should apply for any Linux really.