kirkwood: fix Linksys upgrade, restore config step

kirkwood: fix Linksys upgrade, restore config step

It appears that the refactor of the upgrade process for NAND devices
resulted in the nand_do_upgrade_success step not being called for
devices using the linksys.sh script. As a result, configuration was
not preserved over sysupgrade steps.

This restores the preservation of configs for kirkwood devices using the
linksys.sh script.

Fixes: e25e6d8e5407 ("base-files: fix and clean up nand sysupgrade code")
Fixes: #12298
Signed-off-by: Michael Trinidad <trinidude4@hotmail.com>