Make empty example with u-boot

Author: mekosko

.github/workflows/riscv-rt.yaml

@@ -41,8 +41,10 @@ jobs:
         run: RUSTFLAGS="-C link-arg=-Triscv-rt/examples/device.x" cargo build --package riscv-rt --target ${{ matrix.target }} --example ${{ matrix.example }} --features=single-hart
       - name : Build (v-trap)
         run: RUSTFLAGS="-C link-arg=-Triscv-rt/examples/device.x" cargo build --package riscv-rt --target ${{ matrix.target }} --example ${{ matrix.example }} --features=v-trap
-      - name: Build (all features)
+      - name : Build (all features except u-boot)
         run: RUSTFLAGS="-C link-arg=-Triscv-rt/examples/device.x" cargo build --package riscv-rt --target ${{ matrix.target }} --example ${{ matrix.example }} --features=s-mode,single-hart,v-trap
+      - name : Build (u-boot)
+        run: RUSTFLAGS="-C link-arg=-Triscv-rt/examples/device.x" cargo build --package riscv-rt --target ${{ matrix.target }} --example empty --features=u-boot
 
   # Job to check that all the builds succeeded
   build-check: