oxnas: add platform_pre_upgrade for sysupgrade

We're in process of cleaning nand_do_platform_check. Currently it leaves
a special mark in /tmp/sysupgrade-nand-path triggering some diffent code
path in nand_upgrade_stage1. This can be a bit confusing.

The plan is to have the check function only check the image and nothing
else. Then platform code (platform_pre_upgrade) should trigger NAND
specific upgrade path. This is what this patch implements.

This follows all other targets making it finally possible to change
nand_do_platform_check.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>

SVN-Revision: 47039

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

@@ -15,6 +15,10 @@ platform_check_image() {
 	return $?
 }
 
+platform_pre_upgrade() {
+	nand_do_upgrade $1
+}
+
 disable_watchdog() {
 	killall watchdog
 	( ps | grep -v 'grep' | grep '/dev/watchdog' ) && {