Jetsons come with Ubuntu, and have hardware accelerated access files added to this…then known as “Linux for Tegra” (L4T). The original JTK1 had fastboot boot loader, but all newer generations (including JTK1 after a flash) use U-Boot boot loader.
JetPack is a front end to the flash software and to adding extra packages. It is designed for an x86_64 host running Ubuntu 14.04. Flash software works from any Linux x86_64 host if just doing command line flashing without the extra “goodies” provided by JetPack.
There is no other officially supported operating system on a Jetson. Some third parties have created Android installs, YMMV. Some other Linux distributions are ported, but probably do not have accelerated hardware access (e.g., no GPU and thus no CUDA).