xbps-src: reserve "kernel_hooks_version" variable.

This variable can be used in templates to specify the kernel version
passed to the Vanilla kernel hooks in /etc/kernel.d/<target>.

xbps-src/shutils/metadata_scripts.sh

@@ -76,6 +76,14 @@ UPDATE="\$4"
 #
 _EOF
 
+	#
+	# Handle kernel hooks.
+	#
+	if [ -n "${kernel_hooks_version}" ]; then
+		_add_trigger kernel
+		echo "export kernel_hooks_version=\"${kernel_hooks_version}\"" >> $tmpf
+	fi
+
 	#
 	# Handle DKMS modules.
 	#