Provide an option to rebuild the kernel without downloading it again

Normally, rebuilding the kernel involves removing the previously built kernel
source tree, downloading the stock kernel, applying all the SONiC patches,
and compiling the kernel and the kernel modules. In a development environment,
you might want to skip downloading the kernel and applying all the patches,
and just rebuild the kernel. This option is enabled by setting the value for
DEFAULT_KERNEL_PROCURE_METHOD to "incremental" in the rules/config file.

Procedure to rebuild the kernel:

make target/debs/stretch/linux-headers-4.9.0-9-2-common_4.9.168-1+deb9u5_all.deb-clean
make target/debs/stretch/linux-headers-4.9.0-9-2-common_4.9.168-1+deb9u5_all.deb

You can then upload and install the kernel archive in the switch:
target/debs/stretch/linux-image-4.9.0-9-2-amd64_4.9.168-1+deb9u5_amd64.deb

Makefile

@@ -16,7 +16,7 @@ LINUX_IMAGE = linux-image-$(KVERSION)_$(KERNEL_VERSION)-$(KERNEL_SUBVERSION)_amd
 MAIN_TARGET = $(LINUX_HEADER_COMMON)
 DERIVED_TARGETS = $(LINUX_HEADER_AMD64) $(LINUX_IMAGE)
 
-ifneq ($(kernel_procure_method), build)
+ifeq ($(kernel_procure_method), download)
 # Downloading kernel
 
 # TBD, need upload the new kernel packages
@@ -51,16 +51,29 @@ DSC_FILE_URL = "https://sonicstorage.blob.core.windows.net/packages/kernel-sourc
 DEBIAN_FILE_URL = "https://sonicstorage.blob.core.windows.net/packages/kernel-source/linux_4.9.168-1+deb9u5.debian.tar.xz?sv=2015-04-05&sr=b&sig=koCXHDvmY39smVGcI3cPJrBDZMmdpKiLkyJPfMdNPwU%3D&se=2033-04-28T06%3A14%3A51Z&sp=r"
 ORIG_FILE_URL = "https://sonicstorage.blob.core.windows.net/packages/kernel-source/linux_4.9.168.orig.tar.xz?sv=2015-04-05&sr=b&sig=ArvSGD3N46WGh%2BTYF8J1JgdT9x0BrFu4JhSuyyr3nNw%3D&se=2033-04-09T01%3A00%3A47Z&sp=r"
 
+ifneq ($(wildcard $(BUILD_DIR)),)
+ifeq ($(kernel_procure_method), incremental)
+INCR_BUILD = YES
+endif
+endif
+
 $(addprefix $(DEST)/, $(MAIN_TARGET)): $(DEST)/% :
 	# Obtaining the Debian kernel source
+ifeq ($(INCR_BUILD),)
 	rm -rf $(BUILD_DIR)
 	wget -O $(DSC_FILE) $(DSC_FILE_URL)
 	wget -O $(ORIG_FILE) $(ORIG_FILE_URL)
 	wget -O $(DEBIAN_FILE) $(DEBIAN_FILE_URL)
 
 	dpkg-source -x $(DSC_FILE)
+endif
 
 	pushd $(BUILD_DIR)
+
+ifneq ($(INCR_BUILD),)
+	# Force kernel build
+	rm -f debian/stamps/build_amd64_none_amd64
+else
 	git init
 	git add -f *
 	git commit -qm "check in all loose files and diffs"
@@ -86,6 +99,7 @@ $(addprefix $(DEST)/, $(MAIN_TARGET)): $(DEST)/% :
 	# Learning new git repo head (above commit) by calling stg repair.
 	stg repair
 	stg import -s ../patch/series
+endif
 
 	# Building a custom kernel from Debian kernel source
 	DO_DOCS=False fakeroot make -f debian/rules -j $(shell nproc) binary-indep

README.md

@@ -17,3 +17,15 @@ Usage:
     make DEST=<destination path>
 
 If DEST is not set, package will stay in current directory
+
+## Incremental Kernel Build
+
+Normally, rebuilding the kernel involves removing the previously built kernel source tree, downloading the stock kernel, applying all the SONiC patches, and compiling the kernel and the kernel modules. In a development environment, you might want to skip downloading the kernel and applying all the patches, and just rebuild the kernel. This option is enabled by setting the value for DEFAULT_KERNEL_PROCURE_METHOD to "incremental" in the rules/config file.
+
+Procedure to rebuild the kernel:
+
+    make target/debs/stretch/linux-headers-4.9.0-9-2-common_4.9.168-1+deb9u5_all.deb-clean
+    make target/debs/stretch/linux-headers-4.9.0-9-2-common_4.9.168-1+deb9u5_all.deb
+
+You can then upload and install the kernel archive in the switch:
+target/debs/stretch/linux-image-4.9.0-9-2-amd64_4.9.168-1+deb9u5_amd64.deb