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/imx6/base-files/lib/upgrade/platform.sh

@@ -16,7 +16,7 @@ platform_check_image() {
 	return 1
 }
 
-platform_pre_upgrade() {
+platform_do_upgrade() {
 	local board=$(board_name)
 
 	case "$board" in