diff --git a/build-kernel.sh b/build-kernel.sh
index 7baa491..c3c0fd2 100755
--- a/build-kernel.sh
+++ b/build-kernel.sh
@@ -3,20 +3,21 @@ set -e
 set -x
 
 # Create target dir for build artefacts
-CDIR=$PWD
-BUILD_NR=$(date '+%Y%m%d-%H%M%S')
+WORKDIR=$PWD
+BUILD_NR=${BUILD_NR:=$(date '+%Y%m%d-%H%M%S')}
 BUILD_DEST=/builds/$BUILD_NR
 mkdir -p $BUILD_DEST
 
 # Get the Linux kernel 4.9 source
+BRANCH=rpi-4.9.y
 if [[ -d $LINUX ]]; then
   # update kernel repo
   cd $LINUX
   git pull
-  git checkout rpi-4.9.y
+  git checkout $BRANCH
 else
   # clone kernel repo
-  git clone --single-branch --branch rpi-4.9.y --depth 1 https://www.github.com/raspberrypi/linux $LINUX
+  git clone --single-branch --branch $BRANCH --depth 1 https://www.github.com/raspberrypi/linux $LINUX
   cd $LINUX
 fi
 
@@ -42,7 +43,7 @@ $MAKE
 $MAKE modules
 
 # Install modules
-INSTALLDIR=$CDIR/$KR
+INSTALLDIR=$WORKDIR/$KR
 if [[ -d $INSTALLDIR ]]; then
   rm -fr $INSTALLDIR
   mkdir -p $INSTALLDIR
@@ -59,9 +60,18 @@ cp arch/arm64/boot/Image $INSTALLDIR/boot/kernel8.img
 cp arch/arm64/boot/dts/broadcom/bcm2710-rpi-3-b.dtb $INSTALLDIR/boot/
 cp arch/arm64/boot/dts/overlays/*.dtbo $INSTALLDIR/boot/overlays/
 
-# Create tar file
-tar -cjvf $CDIR/$KR.tar.bz2 -C $INSTALLDIR .
+# Create tar file, all kernel files
+TARFILE1=$KR.tar.gz
+tar -cvzf $WORKDIR/$TARFILE1 -C $INSTALLDIR .
+cd $WORKDIR
+sha256sum $TARFILE1 > $TARFILE1.sha256
+
+# Create tar file, only bootfiles
+TARFILE2=bootfiles.tar.gz
+tar -cvzf $WORKDIR/$TARFILE2 -C $INSTALLDIR/boot .
+cd $WORKDIR
+sha256sum $TARFILE2 > $TARFILE2.sha256
 
 # Copy build artefacts
-cp $CDIR/$KR.tar.bz2 $BUILD_DEST/
-cp -R $INSTALLDIR/boot $BUILD_DEST/
+cp $TARFILE1* $BUILD_DEST/
+cp $TARFILE2* $BUILD_DEST/