scripts/kernel-test: add KERNEL_SKIP_REBUILD

Speed up local test cycles by not building the kernel again from source
if not needed.

Change-Id: Iaa401918d7c09ec92ec546071d8961e99a4cbb47

README.md

@@ -82,6 +82,8 @@ Environment variables:
   repositories in the same local linux clone, default: net-next)
 * `KERNEL_URL`: git remote url (default: net-next.git on kernel.org)
 * `KERNEL_BRANCH` branch to checkout (default: master)
+* `KERNEL_SKIP_REBUILD`: set to 1 to not build the kernel again if already
+  built with `KERNEL_BUILD=1`
 
 The OBS repository mirror consists of
 `${OSMOCOM_REPO_MIRROR}/${OSMOCOM_REPO_PATH}/${OSMOCOM_REPO_VERSION}`,

jenkins-common.sh

@@ -451,6 +451,7 @@ kernel_test_prepare() {
 		-e "KERNEL_CONFIG_BASE=$kernel_config_base" \
 		-e "KERNEL_REMOTE_NAME=$KERNEL_REMOTE_NAME" \
 		-e "KERNEL_URL=$KERNEL_URL" \
+		-e "KERNEL_SKIP_REBUILD=$KERNEL_SKIP_REBUILD" \
 		"$@" \
 		"$docker_image" \
 		"/kernel-test/prepare.sh"

scripts/kernel-test/kernel-build.sh

@@ -61,8 +61,13 @@ update_kernel_config() {
 	cp .config "$previous"
 }
 
-prepare_git_repo
-update_kernel_config
+output="$KERNEL_DIR"/arch/x86/boot/bzImage
 
-make "-j$(nproc)"
-cp arch/x86/boot/bzImage /cache/kernel-test/linux
+if [ ! -e "$output" ] || [ "$KERNEL_SKIP_REBUILD" != 1 ]; then
+	prepare_git_repo
+	update_kernel_config
+
+	make "-j$(nproc)"
+fi
+
+cp "$output" /cache/kernel-test/linux