There's not, and it's also not supported officially. Multi-boot needs to be implemented first.
I'm reluctant to offer instructions because Planet hasn't given me the all-clear to do so, nor is there any stock firmware available for downloading and flashing onto the Astro should the process go wrong.
I realise this is frustrating but be assured Linux support is on the way.
I stress the importance of not messing with firmware, rooting, or other firmware mods unless you have a FULL backup (including NVRAM) and know what you're doing.