ci: try to build bl2 with ubi support for R4

Author: frank-w

.github/workflows/build.yml

@@ -215,6 +215,9 @@ jobs:
           echo -e "board=bpi-r4\ndevice=spim-nand" > build.conf
           bash build.sh build
           bash build.sh rename
+          echo -e "extraflags=UBI=1" >> build.conf
+          bash build.sh build
+          cp build/mt7988/release/bl2.img bpi-r4_spim-nand_ubi_bl2.img
           rm build.conf
 
       - name: Upload binaries to release