Add single-kernel workflow

In some cases it might be handy to quickly smoke test certain types of
changes, where it usually doesn't make sense to build everything.

So lets introduce a workflow which can build a kernel for single
target/subtarget.

References: openwrt/openwrt#16784
Signed-off-by: Petr Štetiar <ynezz@true.cz>

Author: ynezz

.github/workflows/single-kernel.yml

@@ -0,0 +1,51 @@
+name: Build single kernel with external toolchain and check patches
+on:
+  workflow_call:
+    inputs:
+      container_name:
+        type: string
+        default: toolchain
+      target:
+        required: true
+        type: string
+      subtarget:
+        required: true
+        type: string
+      testing:
+        type: boolean
+      build_all_modules:
+        type: boolean
+      build_all_kmods:
+        type: boolean
+      build_dtb:
+        type: boolean
+
+jobs:
+  build_kernel:
+    name: Build kernel with external toolchain
+    permissions:
+      contents: read
+      packages: read
+      actions: write
+    uses: ./.github/workflows/reusable_build.yml
+    with:
+      container_name: ${{ inputs.container_name }}
+      target: ${{ inputs.target }}
+      subtarget: ${{ inputs.subtarget }}
+      testing: ${{ inputs.testing != '' && true }}
+      build_kernel: true
+      build_all_modules: ${{ inputs.build_all_modules != '' && true }}
+      build_all_kmods: ${{ inputs.build_all_kmods != '' && true }}
+      build_dtb: ${{ inputs.build_dtb != '' && true }}
+
+  check-kernel-patches:
+    name: Check kernel patches
+    permissions:
+      contents: read
+      packages: read
+      actions: write
+    uses: ./.github/workflows/reusable_check-kernel-patches.yml
+    with:
+      target: ${{ inputs.target }}
+      subtarget: ${{ inputs.subtarget }}
+      testing: ${{ inputs.testing != '' && true }}