I have checked old SDKs.

SDK v0.9.2 (20 Oct 2017): no "-o" option for bossac.
SDK v0.9.1 (20 May 2017): no "-o" option for bossac.

The "-o" for bossac.py is added on 9 Jul 2018
https://github.com/zephyrproject-rtos/zephyr/commit/bde3224c911176a6af4fa60a067c7492dfc76fe4

I just wonder which environment they did check.

It may be possible to replace bossac binary with "-o" option, but I am not sure whether it is best way for Zephyr.
I will just check further information myself, but for a while, I give up using Adafruit Trinket M0 and will use other boards.