target/x86: support custom grub hook

target/linux/x86/64/base-files/lib/upgrade/keep.d/grub

@@ -0,0 +1,2 @@
+/boot/grub/grub.ext.cfg
+/boot/grub/grubenv

target/linux/x86/64/base-files/usr/sbin/update-grub

@@ -0,0 +1,20 @@
+#!/bin/sh
+
+GRUB_CFG=/boot/grub/grub.ext.cfg
+
+merge_cfg() {
+	local cfgfile
+	echo "# Merged /etc/grub.cfg.d/*.cfg"
+	echo "# WARNING: Do not edit this file by tools other than update-grub!!!"
+	ls /etc/grub.cfg.d/*.cfg | while read; do
+		cfgfile="$REPLY"
+		echo "# $cfgfile"
+		cat "$cfgfile"
+		echo
+	done
+}
+
+exec 512>/var/lock/update-grub.lock
+flock 512 || exit 1
+
+merge_cfg >$GRUB_CFG