realtek-poe: build default config from board.json

realtek-poe: build default config from board.json

This adjustment makes a compromise between those that would prefer not
to complicate the realtek-poe.git source repository and those who want
to have PoE work out-of-the-box on Realtek switches -- see
realtek-poe.git, commit 2df137ee457155 ("realtek-poe/openwrt: Generate
config from board.json")

Switches previously booted with OpenWrt will have an /etc/config/poe
file already present, and thus the uci-default shall not take effect.

Signed-off-by: Martin Kennedy <redacted>