ath79: read label MAC address from flash instead of using phy0/phy1

ath79: read label MAC address from flash instead of using phy0/phy1

This replaces all uses of $(cat /sys/class/ieee80211/phyX/macaddress)
by retrieval from the proper flash locations. This will make
02_network independent of WiFi setup again.

For future reference:

The MAC addresses of the FRITZ!WLAN Repeater 300E can be extracted
from the urlader key value store:

maca        *:6B
macb        *:6C
macwlan     *:6D
macdsl      *:6E

and be set by e.g.
addr=$(fritz_tffs -n maca -i $(find_mtd_part "tffs (1)"))

Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>