I don't see why not. The default for shared_buffers is 32 MB, and can be set as low as 16 pages (128 KB). Assuming you only need a few MB for work_mem and wal_buffers. Watch out for maintenance_work_mem as this number gets multiplied times the number of workers (defaults to 3) times the number of concurrent sessions requesting maintenance work memory (rebuilding indexes, etc.).
I have not tried it on an embedded device, but I assume it works the same.
Sorry, but i believe it all depends on usage, i mean just for a start and shutdown procedure it may work, but 128mb in any work environment is not even enough for bgworkers that autovaccum tables, maintenance of cronjobs, and if the DB is accessed through a application then that is another overhead, i think you will find yourself short of memory. So best is evaluate USAGE rather than STARTUP load