Test with const-fn on nightly

riscv-rt/ci/script.sh

@@ -4,7 +4,7 @@ main() {
     cargo check --target $TARGET
 
     if [ $TRAVIS_RUST_VERSION = nightly ]; then
-        cargo check --target $TARGET --features inline-asm
+        cargo check --target $TARGET --features 'const-fn inline-asm'
     fi
 }