ipq806x: Fix Linksys upgrade, restore config step

ipq806x: 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 ipq806x devices using the
linksys.sh script. Other devices and targets have not been examined.

This commit uses the same functionality and terminology used in commit
8634c10 ("ipq40xx: Fix Linksys upgrade, restore config step")

Fixes: e25e6d8 ("base-files: fix and clean up nand sysupgrade code")
Tested-on: EA8500

Signed-off-by: Jacob Aharon <ah.jacob@gmail.com>