feat: provide an easy way to manage kernel configuration

This automates the kernel configuration generation process described in
`kernel/README.md` and wraps it as easy to use `make` targets, e.g.:

* `make kernel-olddefconfig` updates kernel config for the new kernel
version
* `make kernel-menuconfig` launches interactive kernel configuration

List of architectures can be updated via `PLATFORM=` variable override.

Signed-off-by: Andrey Smirnov <smirnov.andrey@gmail.com>

Makefile

@@ -13,6 +13,10 @@ COMMON_ARGS := --file=Pkgfile
 COMMON_ARGS += --progress=$(PROGRESS)
 COMMON_ARGS += --platform=$(PLATFORM)
 
+, := ,
+space :=
+space +=
+
 TARGETS =  ca-certificates  cni  containerd  dosfstools  eudev  fhs  grub  ipmitool  iptables  kernel  kmod  libaio  libressl  libseccomp  linux-firmware lvm2  musl  open-iscsi  open-isns  runc  socat  syslinux  util-linux  xfsprogs
 
 all: $(TARGETS) ## Builds all known pkgs.
@@ -37,3 +41,10 @@ $(TARGETS):
 .PHONY: deps.png
 deps.png:
 	bldr graph | dot -Tpng > deps.png
+
+kernel-%: ## Updates the kernel configs: e.g. make kernel-olddefconfig; make kernel-menuconfig; etc.
+	for platform in $(subst $(,),$(space),$(PLATFORM)); do \
+		arch=`basename $$platform` ; \
+		$(MAKE) docker-kernel-prepare PLATFORM=$$platform TARGET_ARGS="--tag=$(REGISTRY)/$(USERNAME)/kernel:$(TAG)-$$arch --load"; \
+		docker run --rm -it --entrypoint=/toolchain/bin/bash -e PATH=/toolchain/bin:/bin -w /src -v $$PWD/kernel/kernel/config-$$arch:/src/.hostconfig $(REGISTRY)/$(USERNAME)/kernel:$(TAG)-$$arch -c 'cp .hostconfig .config && make $* && cp .config .hostconfig'; \
+	done

kernel/README.md

@@ -1,32 +1,28 @@
 # Kernel
 
-## Customizing the kernel
+## Updating kernel config
+
+When updating kernel to the new version, import proper defaults with:
 
-High-level notes:
+```sh
+make kernel-olddefconfig
+```
 
-- In pkg.yaml, comment out the `build` and `install` steps
-- At the bottom of pkg.yaml, change the finalize step from
+If you want to update for a specific architecture only, use:
 
-```yaml
-finalize:
-  - from: /rootfs
-    to: /
+```sh
+make kernel-olddefconfig PLATFORM=linux/arm64
 ```
 
-to
+## Customizing the kernel
+
+Run another target to get into `menuconfig`:
 
-```yaml
-finalize:
-  - from: /
-    to: /
+```sh
+make kernel-menuconfig
 ```
 
-- Create a local image with `docker buildx build -t kernel --target kernel -f Pkgfile --load .`
-- Run the kernel image we created: `docker run --rm -it --entrypoint=/toolchain/bin/bash kernel`
-- Set path: `export PATH=/toolchain/bin:/bin`
-- Change to build dir: `cd /tmp/build/0`
-- Make changes to kernel settings with `make menuconfig` and save upon exiting
-- With the container still running, copy the config out to local disk: `docker cp $CONTAINER_ID:/tmp/build/0/.config config-amd64`
-- Revert your changes to pkg.yaml
+## Testing
+
 - Build and push a test image with `make USERNAME=rsmitty PUSH=true kernel`
 - PR upstream (when ready) and profit

kernel/kernel-prepare/pkg.yaml

@@ -0,0 +1,57 @@
+name: kernel-prepare
+variant: scratch
+shell: /toolchain/bin/bash
+dependencies:
+  - image: '{{ .TOOLS_IMAGE }}'
+steps:
+  - sources:
+      - url: https://cdn.kernel.org/pub/linux/kernel/v5.x/linux-5.7.7.tar.xz
+        destination: linux.tar.xz
+        sha256: f840b9679283343c165516585c3070ebb277528721c890e9410a58e9d071ee7f
+        sha512: 49db85a1ce14e00411215d5d5bfda6db3d24ed2e0f2bd8e6603c18b3226614f45040856b21b4d6b525c44bb4463ab08fb594a06de6deca109d512588389cc3fd
+    env:
+      ARCH: {{ if eq .ARCH "aarch64"}}arm64{{ else if eq .ARCH "x86_64" }}x86_64{{ else }}unsupported{{ end }}
+    prepare:
+      - |
+        tar -xJf linux.tar.xz --strip-components=1
+        rm linux.tar.xz
+
+        mkdir /bin
+        ln -sv /toolchain/bin/bash /bin/bash
+        ln -sv /toolchain/bin/bash /bin/sh
+        ln -sv /toolchain/lib /lib
+        mkdir -p /usr/bin \
+            && ln -sf /toolchain/bin/env /usr/bin/env \
+            && ln -sf /toolchain/bin/true /bin/true \
+            && ln -sf /toolchain/bin/false /bin/false \
+            && ln -sf /toolchain/bin/pwd /bin/pwd
+
+        # Ensure that `make menuconfig` works.
+
+        ln -s /toolchain/bin/awk /usr/bin/awk
+
+        mkdir -p /usr/lib/pkgconfig
+        ln -s /toolchain/include /usr/include
+
+        for lib in ncurses form panel menu ; do
+            rm -vf                    /lib/lib${lib}.so
+            echo "INPUT(-l${lib}w)" > /lib/lib${lib}.so
+            ln -sfv ${lib}w.pc        /lib/pkgconfig/${lib}.pc
+        done
+
+        make mrproper
+    install:
+      - |
+        mkdir -p /src
+        cp -a . /src/
+finalize:
+  - from: /src
+    to: /src
+  - from: /toolchain
+    to: /toolchain
+  - from: /usr
+    to: /usr
+  - from: /bin
+    to: /bin
+  - from: /lib
+    to: /lib

kernel/kernel/pkg.yaml

@@ -0,0 +1,44 @@
+name: kernel
+variant: scratch
+shell: /toolchain/bin/bash
+dependencies:
+  - stage: kernel-prepare
+steps:
+  - env:
+      ARCH: {{ if eq .ARCH "aarch64"}}arm64{{ else if eq .ARCH "x86_64" }}x86_64{{ else }}unsupported{{ end }}
+      CARCH: {{ if eq .ARCH "aarch64"}}arm64{{ else if eq .ARCH "x86_64" }}amd64{{ else }}unsupported{{ end }}
+    prepare:
+      - |
+        cp -a /src/. .
+
+        cp -v /pkg/config-${CARCH} .config
+    build:
+      - |
+        make -j $(nproc)
+        make -j $(nproc) modules
+    install:
+      - |
+        mkdir -p /rootfs/boot
+        case $ARCH in
+            x86_64)
+                mv arch/x86/boot/bzImage /rootfs/boot/vmlinuz
+                mv vmlinux /rootfs/boot/vmlinux
+            ;;
+            arm64)
+                mv arch/arm64/boot/Image.gz /rootfs/boot/vmlinuz
+                mv vmlinux /rootfs/boot/vmlinux
+            ;;
+            *)
+                echo "unsupported arch ${ARCH}"
+                exit 1
+            ;;
+        esac
+
+        export KERNELRELEASE=$(cat include/config/kernel.release)
+        make -j $(nproc) modules_install DEPMOD=/toolchain/bin/depmod INSTALL_MOD_PATH=/rootfs
+        depmod -b /rootfs $KERNELRELEASE
+        unlink /rootfs/lib/modules/$KERNELRELEASE/build
+        unlink /rootfs/lib/modules/$KERNELRELEASE/source
+finalize:
+  - from: /rootfs
+    to: /