This is what I did: Use the efidroid lk.img (have to compile yourself right now) for working around an fastboot/lk issue in the stock Fairphone bootloader.
fastboot boot lk.img
fastboot flash system pmOS_system.img
Booting the boot.img doesn't work currently because of dt problems. Probably a similar issue as on the Mozilla Flame.
pmOS branch: https://github.com/postmarketOS/pmbootstrap/tree/device-fairphone-fp2 z3ntu's branch (maybe more up-to-date): https://github.com/z3ntu/pmbootstrap
lk = Little Kernel (that's what the bootloader which includes fastboot is based on for most newer devices)
dt = Device Tree (method of describing hardware, see https://www.devicetree.org/)