Add workflow for automating kernel build

This change automates kernel builds on our self-hosted runner.

.github/workflows/build.yml

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+name: Kernel Build
+
+on:
+  push:
+    branches: [master]
+    tags:
+        - release/*
+
+jobs:
+  build:
+    runs-on: [self-hosted]
+    steps:
+    - name: Checkout Sources
+      uses: actions/checkout@v2
+      with:
+          clean: false # Re-use artifacts from previous build
+          submodules: recursive
+    - name: Build Kernel
+      run: make
+    - name: Upload Artifacts
+      uses: actions/upload-artifact@v2-preview
+      with:
+        name: debs
+        path: "*.deb"
+

README.md

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+# Proxmox Edge kernels
+Custom Linux kernels for Promox VE 6.