jenkins.sh: change qemu-img default location to $HOME/qemu-img instead of /opt/qemu-img

Change-Id: I56f314d78c0ca968b1fef9a91ecd540a7cc8fa86
diff --git a/contrib/jenkins.sh b/contrib/jenkins.sh
index 10fc2b1..c4d786d 100755
--- a/contrib/jenkins.sh
+++ b/contrib/jenkins.sh
@@ -28,7 +28,7 @@
         if ! $(substr "arm" "$(uname -m)") && [ "x${INSTR}" = "x--with-neon" -o "x${INSTR}" = "x--with-neon-vfpv4" ]; then
 
                 OSMOTRX_DIR="$PWD" # we assume we are called as contrib/jenkins.sh
-                ROOTFS_PREFIX="${ROOTFS_PREFIX:-/opt}"
+                ROOTFS_PREFIX="${ROOTFS_PREFIX:-$HOME}"
                 ROOTFS="${ROOTFS_PREFIX}/qemu-img"
                 mkdir -p "${ROOTFS_PREFIX}"