image-sdk: Pull the Mesa drivers linking to the LGPL-3 libelf

With pkg/target/mesa!3 Mesa ships the llvmpipe enabled swrast driver again but, to avoid GPL-3 issues due to the linkage with the GPL-2/LGPL-3 libelf library, it has been moved to the libgl1-mesa-dri-llvm package.

Install it so GL still works on the SDK images, letting people run mildenhall-compositor --nested --wayland again.