gluon-core: read NanoPi R2S primary MAC from LAN board data

The relevant entry for the primary MAC location was lost when rebasing
the patch on OpenWrt 21.02.

Fixes commit ded4b8a7 ("rockchip-armv8: add FriendlyARM NanoPi R2S")

Signed-off-by: David Bauer <mail@david-bauer.net>