sys-kernel/coreos-modules: build AMD GPU as module

[tormath1]

Related to: https://github.com/flatcar/Flatcar/issues/1935

• [x] Changelog entries added in the respective changelog/ directory (user-facing change, bug fix, security fix, update)
• [ ] Inspected CI output for image differences: /boot and /usr size, packages, list files for any missing binaries, kernel modules, config files, kernel modules, etc.
• [ ] http://localhost:8080/job/container/job/packages_all_arches/6922/cldsv/

Currently failing with:

11:46:46   [31;01m*[0m Missing firmware: amdgpu/ip_discovery.bin (amdgpu.ko.xz)
11:46:46   [31;01m*[0m Missing firmware: amdgpu/vega10_cap.bin (amdgpu.ko.xz)
11:46:46   [31;01m*[0m Missing firmware: amdgpu/sienna_cichlid_cap.bin (amdgpu.ko.xz)
11:46:46   [31;01m*[0m Missing firmware: amdgpu/navi12_cap.bin (amdgpu.ko.xz)
11:46:46   [31;01m*[0m Missing firmware: amdgpu/aldebaran_cap.bin (amdgpu.ko.xz)
11:46:46   [31;01m*[0m Missing firmware: amdgpu/gc_11_0_0_toc.bin (amdgpu.ko.xz)
11:46:46   [31;01m*[0m Missing firmware: amdgpu/gc_11_0_3_mes.bin (amdgpu.ko.xz)

I need to check what's going on the firmware (maybe some symlinks are required)