scripts: Add prepare_linux function

`prepare_linux` checks if a `--build-guest-kernel` option is present,
and build kernel from `cloud-hypervisor/linux.git`. Otherwise, it will
invoke `download_linux` to use pre-built kernel.

Signed-off-by: Ruoqing He <heruoqing@iscas.ac.cn>

scripts/test-util.sh

@@ -1,6 +1,7 @@
 #!/usr/bin/env bash
 hypervisor="kvm"
 test_filter=""
+build_kernel=false
 
 # Checkout source code of a GIT repo with specified branch and commit
 # Args:
@@ -75,11 +76,13 @@ cmd_help() {
     echo ""
     echo "    --hypervisor  Underlying hypervisor. Options kvm, mshv"
     echo "    --test-filter Tests to run"
+    echo "    --build-guest-kernel Build guest kernel from source instead of downloading pre-built"
     echo ""
     echo "    --help        Display this help message."
     echo ""
 }
 
+# shellcheck disable=SC2034
 process_common_args() {
     while [ $# -gt 0 ]; do
         case "$1" in
@@ -93,9 +96,11 @@ process_common_args() {
             ;;
         "--test-filter")
             shift
-            # shellcheck disable=SC2034
             test_filter="$1"
             ;;
+        "--build-guest-kernel")
+            build_kernel=true
+            ;;
         "--") {
             shift
             break
@@ -149,6 +154,18 @@ download_linux() {
     popd || exit
 }
 
+prepare_linux() {
+    if [ "$build_kernel" = true ]; then
+        echo "Building kernel from source"
+        build_custom_linux
+        echo "Using kernel built from source"
+    else
+        echo "Downloading pre-built kernel from GitHub"
+        download_linux
+        echo "Using kernel downloaded from GitHub"
+    fi
+}
+
 download_ovmf() {
     OVMF_FW_TAG="ch-6624aa331f"
     OVMF_FW_URL="https://github.com/cloud-hypervisor/edk2/releases/download/$OVMF_FW_TAG/CLOUDHV.fd"