Kernel: Bake version information into the Kernel

This is done by generating a Kernel/Version.h header with major version,
minor version, and git hash.

Kernel/generate-version-file.sh

@@ -0,0 +1,37 @@
+#!/bin/sh
+
+set -e
+
+OUTPUT_FILE=$1
+
+if command -v git >/dev/null; then
+    if git status >/dev/null 2>&1; then
+        GIT_HASH=$( (git log --pretty=format:'%h' -n 1 | cut -c1-7) || true )
+        # There is at least one modified file as reported by git.
+        if git status --porcelain=v2 | head | grep -Ei '^1' >/dev/null; then
+            GIT_HASH="${GIT_HASH}-modified"
+        fi
+    else
+        GIT_HASH=unknown
+    fi
+else
+    GIT_HASH=unknown
+fi
+
+
+cat << EOF > "$OUTPUT_FILE"
+/*
+ * Automatically generated by Kernel/generate-version-file.sh
+ */
+
+#pragma once
+#include <AK/StringView.h>
+
+namespace Kernel {
+
+constexpr unsigned SERENITY_MAJOR_REVISION = 1;
+constexpr unsigned SERENITY_MINOR_REVISION = 0;
+constexpr StringView SERENITY_VERSION = "${GIT_HASH}"sv;
+
+}
+EOF