I have a fresh install of OpenWrt 15.05-rc2 on a RaspBerry B+
root@bnh-traveller:~# uname -a
Linux bnh-traveller 3.18.11 #1 Mon May 18 21:56:20 CEST 2015 armv6l GNU/Linux
I have already installed required packages with success
root@bnh-traveller:~# opkg install kmod-rt2800-lib kmod-rt2800-usb kmod-rt2x00-lib kmod-rt2x00-usb
Package kmod-rt2800-lib (3.18.14+2015-03-09-3) installed in root is up to date.