ar71xx: ubnt-(xm,xw): add rssileds package

ar71xx: ubnt-(xm,xw): add rssileds package

In order to make RSSI indicator on the device work out of box,
include "rssileds" package in per-device rootfs image by default
for Ubiquiti XM and XW devices, namely:
- Bullet M (XM/XW)
- Rocket M (XM/XW)
- Nanostation M (XM/XW)
- Nanostation Loco-M (XW)

This moves the package addition to the individual devices in order
to prevent accidental inclusions of the package when not looking
at the parent node carefully enough.

Signed-off-by: Lech Perczak <lech.perczak@gmail.com>
[add bullet-m-xw, remove rocket-m-ti, extend commit message]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>