kbuild: rpm-pkg: introduce a simple changelog section for kernel.spec

Fix the following rpmbuild warning:

$ make srcrpm-pkg
...
RPM build warnings:
source_date_epoch_from_changelog set but %changelog is missing

Signed-off-by: Rafael Aquini <[e-mail]>
Signed-off-by: Masahiro Yamada <[e-mail]>

scripts/package/mkspec

···
 EOF
 
 cat "${srctree}/scripts/package/kernel.spec"
+
+# collect the user's name and email address for the changelog entry
+if [ "$(command -v git)" ]; then
+	name=$(git config user.name) || true
+	email=$(git config user.email) || true
+fi
+
+if [ ! "${name:+set}" ]; then
+	name=${KBUILD_BUILD_USER:-$(id -nu)}
+fi
+
+if [ ! "${email:+set}" ]; then
+	buildhost=${KBUILD_BUILD_HOST:-$(hostname -f 2>/dev/null || hostname)}
+	builduser=${KBUILD_BUILD_USER:-$(id -nu)}
+	email="${builduser}@${buildhost}"
+fi
+
+cat << EOF
+
+%changelog
+* $(LC_ALL=C; date +'%a %b %d %Y') ${name} <${email}>
+- Custom built Linux kernel.
+EOF