Fixed checkout of proper u-boot version

Signed-off-by: Dominik Adamski <adamski.dominik@gmail.com>

rpi3-convert-stage-5.sh

@@ -50,11 +50,12 @@ build_uboot_files() {
 
   if [ ! -d $uboot_repo_vc_dir ]; then
     git clone https://github.com/mendersoftware/uboot-mender.git -b $branch
-    git checkout $commit
   fi
 
   cd $uboot_dir
 
+  git checkout $commit
+
   make --quiet distclean
   make --quiet rpi_3_32b_defconfig && make --quiet
   make envtools