Change brcmfmac to kernel module from built-in
Change brcmfmac to kernel module from built-in

The rpi3 system includes brcmfmac as a module, and it's confusing to people who go between devices that the rpi0 system includes it as a built in kernel driver. This change makes the two systems consistent.

nerves-project/nerves_system_rpi0,nerves-project/nerves_system_rpi0