base-files: move uci_set_poe() to uci-defaults.sh
PoE devices in the realtek target have the possibility to add PSE info
to the board description via 02_network. Make this available for all
targets, by moving the uci_set_poe() function to the globally available
uci-default.sh script.
Signed-off-by: Sander Vanheule <[email protected]>