ath79: engenius epg5000: add leds migration script

ath79: engenius epg5000: add leds migration script

With transition from ar71xx to ath79 some of devices change their naming
of LEDs. When upgrading from ar71xx target images this will require the
user to adjust previously working configuration. This commit adds
migration script which can be used to rename old names to new ones.
With this previously working configuration will be automatically
adjusted, wihtout user intervention.

This commit adds migration case for EnGenius EPG5000, the wireless LEDs
names have changed from epg5000:blue:wlan2-g and epg5000:blue:wlan-5g to
epg5000:blue:wlan2g and epg5000:blue:wlan5g.

Signed-off-by: Tomasz Maciej Nowak <tomek_n@o2.pl>