treewide: move nand_do_upgrade call to platform_do_upgrade

Calling nand_do_upgrade() from platform_pre_upgrade() was deprecated
with 30f61a34b4 ("base-files: always use staged sysupgrade").

Update the platform upgrade code to use platform_do_upgrade() for NAND
images as well.

Signed-off-by: Mathias Kresin <dev@kresin.me>

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

@@ -13,6 +13,6 @@ platform_check_image() {
 	return $?
 }
 
-platform_pre_upgrade() {
+platform_do_upgrade() {
 	nand_do_upgrade $1
 }