mvebu: fix Turris MOX update

Turris MOX still upgrades from a legacy SD card image rather than
using the default sysupgrade path for cortexa53 devices.

Route image validation, upgrade handling and config backup through
the legacy_sdcard helpers so sysupgrade writes the correct image
format and preserves settings.

Signed-off-by: Štěpán Dalecký <daleckystepan@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/23666
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
(cherry picked from commit d21b7bfbff)
Signed-off-by: Tomas Macholda <tomas.macholda@nic.cz>
Link: https://github.com/openwrt/openwrt/pull/24442
Signed-off-by: Jonas Jelonek <jelonek.jonas@gmail.com>

target/linux/mvebu/cortexa53/base-files/lib/upgrade/platform.sh

@@ -43,6 +43,7 @@ methode_update_active_bootscript() {
 
 platform_check_image() {
 	case "$(board_name)" in
+	cznic,turris-mox|\
 	glinet,gl-mv1000|\
 	globalscale,espressobin|\
 	globalscale,espressobin-emmc|\
@@ -59,6 +60,7 @@ platform_check_image() {
 
 platform_do_upgrade() {
 	case "$(board_name)" in
+	cznic,turris-mox|\
 	glinet,gl-mv1000|\
 	globalscale,espressobin|\
 	globalscale,espressobin-emmc|\
@@ -117,6 +119,7 @@ platform_do_upgrade() {
 }
 platform_copy_config() {
 	case "$(board_name)" in
+	cznic,turris-mox|\
 	glinet,gl-mv1000|\
 	globalscale,espressobin|\
 	globalscale,espressobin-emmc|\