The PicoStation M2 are supported by OpenWrt.
Picostation M2 device uses Bullet M2 firmware image
/!\ WARNING /!\
airOS firmware versions >5.6 have a new bootloader with an incompatible partition table!
If you install openWRT over these versions, all will seem well, but your configs will not persist after reboot.
To avoid this issue:
If you do not do this, and instead attempt to install openwrt over the top of airOS >5.6 and then, realising your mistake, attempt to downgrade to ≤ 5.5, you will find your device soft-bricked, stuck in a boot loop. This is because the TFTP flashing procedure does not overwrite the bootloader, leaving you with a 5.6 bootloader trying to load a 5.5 firmware, which wont work. Attempts to TFTP other openwrt versions at this point will simply be rejected by the tftp server.
If you find yourself in this situation, you will need to access the serial console using a TTY cable as described here: https://community.ubnt.com/t5/airOS-SDK-Custom-Development/AirMAX-Device-Firmware-Recovery-Procedure/td-p/1499583
The above url details a procedure which can be carried out using the tty console which will allow reflashing the entire image, including the bootloader.
The instructions are mirrored here in case the ubnt forum disappears: http://pastebin.com/raw/FLjgY074
More information is available in the following threads:
https://community.ubnt.com/t5/airOS-Software-Configuration/Cannot-downgrade-firmware-from-5-6-3/td-p/1450154 https://community.ubnt.com/t5/airOS-SDK-Custom-Development/5-6-2-TFTP-ERROR/td-p/1307614 https://community.ubnt.com/t5/UniFi-Wireless/HOWTO-Unbrick-your-UniFi-AP/td-p/338684
The OpenWrt buildroot generates images that can be directly flashed to the Picostation M2 trough the Ubiquiti web, usually with a name like
openwrt-ar71xx-generic-ubnt-bullet-m-squashfs-factory.bin (image for Ubiquiti Bullet M2).
Firmware download links: