ath79: base-files: disable sysupgrade for routerstations and ja76pf2

Previous commit 0cc87b3 "ath79: image: disable sysupgrade images for
routerstations and ja76pf2" doesn't remedy completely the posibility of
bricking the device, since user could try to downgrade with an older
image. Therefore disable sysupgrade code for these boards with a small
note.

Signed-off-by: Tomasz Maciej Nowak <tomek_n@o2.pl>

target/linux/ath79/base-files/lib/upgrade/platform.sh

@@ -45,11 +45,11 @@ platform_do_upgrade() {
 		redboot_fis_do_upgrade "$1" vmlinux_2
 		;;
 	jjplus,ja76pf2)
-		redboot_fis_do_upgrade "$1" linux
+		echo "Sysupgrade disabled due bug FS#2428"
 		;;
 	ubnt,routerstation|\
 	ubnt,routerstation-pro)
-		redboot_fis_do_upgrade "$1" kernel
+		echo "Sysupgrade disabled due bug FS#2428"
 		;;
 	*)
 		default_do_upgrade "$1"