Created attachment 8901 [details] the patch BR2_PACKAGE_SDL2_KMSDRM option is currently broken. Enabling it doesn't do anything useful because kmsdrm driver in sdl2 have more deps. Namely, it needs BR2_PACKAGE_MESA3D_GBM and BR2_PACKAGE_MESA3D_OPENGL_EGL. Additionally it needs the patch that I extracted from sdl git, to even build successfully. See "hg diff -r 14526..14527" in sdl sources. That patch is attached.
I think this was fixed by 6d664e9cfe and a new sdl.