ac-os: pass USE_SDL=1 to make, restore nomodeset for now
build_binary was not passing USE_SDL=1, so the binary had no SDL3
support compiled in. Without SDL3 and without nomodeset, i915 KMS
takes over but DRM dumb buffers fail → black screen.
Restore nomodeset so DRM dumb buffers work via efifb. Keep quiet
loglevel=3 for clean direct-boot. SDL3 will handle KMS when it's
actually compiled in (USE_SDL=1 now passed to make).
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>