How to compile a working kernel?

[Lili1228]

inb4 duplicate of #30
I want to add some modules to the kernel and in order to do that, I need to enable the module support first, which requires compiling the kernel (3.x stable in my case) from the ground up. Kernel built on Ubuntu 16.04 with GCC 4.8 from wii-mini-mode_defconfig preset starts, but hangs on a Tux. With https://github.com/DeltaResero/GC-Wii-Linux-Kernels/blob/master/build-gc-wii-kernel.sh script, by choosing wii_defconfig, I can see in dmesg that it tries every minute to load b43 firmware and practically hangs when it fails for the 4th time. Quite frankly, I don't really understand how it's unable to find it since it's the same kernel revision, files in /lib/firmware/b43 exist and the release kernel works just fine.
So my question is: what am I doing wrong here?