I've been playing around with custom built OpenPower firmware. One tips I pick up from Stewart Smith is to use `mbox` to load the firmware from file instead of mtd:
# SSH into OpenBMC
# Load firmware from file
$ mboxctl --backend file:/tmp/blackbird.pnor
$ obmcutil poweron
# once done with testing
$ obmcutil poweroff
$ mboxctl --backend mtd