ipq806x/nbg6817: add sysupgrade support

Add new way of flashing to mmc devices based on rootfs split with loop devices.

Signed-off-by: André Valentin <avalentin@marcant.net>

target/linux/ipq806x/base-files/lib/upgrade/platform.sh

@@ -9,6 +9,7 @@ platform_check_image() {
 	ap148 |\
 	d7800 |\
 	ea8500 |\
+	nbg6817 |\
 	r7500 |\
 	r7500v2 |\
 	r7800)
@@ -34,6 +35,7 @@ platform_pre_upgrade() {
 	case "$board" in
 	ap148 |\
 	d7800 |\
+	nbg6817 |\
 	r7500 |\
 	r7500v2 |\
 	r7800)
@@ -60,6 +62,16 @@ platform_do_upgrade() {
 	esac
 }
 
+platform_nand_pre_upgrade() {
+	local board=$(ipq806x_board_name)
+
+	case "$board" in
+	nbg6817)
+		zyxel_do_upgrade "$1"
+		;;
+	esac
+}
+
 blink_led() {
 	. /etc/diag.sh; set_state upgrade
 }