treewide: sysupgrade: get rid of platform_nand_pre_upgrade()

1) nand_do_upgrade() is always called by a target code
2) nand_do_upgrade() starts with calling platform_nand_pre_upgrade()

It means there is no need for the platform_nand_pre_upgrade() callback
at all. All code that was present there could bo moved & simplly called
by a target right before the nand_do_upgrade().

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>

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

@@ -21,10 +21,12 @@ platform_do_upgrade() {
 	netgear,r7500v2 |\
 	netgear,r7800 |\
 	qcom,ipq8064-ap148 |\
-	qcom,ipq8064-ap161 |\
-	zyxel,nbg6817)
+	qcom,ipq8064-ap161)
 		nand_do_upgrade "$1"
 		;;
+	zyxel,nbg6817)
+		zyxel_do_upgrade "$1"
+		;;
 	linksys,ea8500)
 		platform_do_upgrade_linksys "$1"
 		;;
@@ -44,11 +46,3 @@ platform_do_upgrade() {
 		;;
 	esac
 }
-
-platform_nand_pre_upgrade() {
-	case "$(board_name)" in
-	zyxel,nbg6817)
-		zyxel_do_upgrade "$1"
-		;;
-	esac
-}

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

@@ -118,5 +118,7 @@ zyxel_do_upgrade() {
 
 	zyxel_do_flash $tar_file $kernel $rootfs $dualflagmtd
 
+	nand_do_upgrade "$1"
+
 	return 0
 }