Step 1: Install uhttpd

cd /tmp
wget http://downloads.openwrt.org/backfire/10.03/atheros/packages/uhttpd_7_atheros.ipk
opkg install /tmp/uhttpd_7_atheros.ipk

Step 2: rc.local

Then edit your /etc/rc.local

root@OpenWrt:/tmp/speed# cat /etc/rc.local 
ifconfig eth0 192.168.150.50
ifconfig eth0:1 192.168.1.1
babeld -z 2,30 eth0 ath0 &
mkdir -p /tmp/speed
dd if=/dev/zero of=/tmp/speed/4M bs=1M count=4 &
uhttpd -h /tmp/speed/ -p 80
exit 0

Then at reboot, it should serve a 4MB file on all its interfaces.

Then, on the other side, do a:

while true; do wget http://192.168.40.50/4M -0 /dev/null; done

The uhttpd still takes some 90% CPU if you download the file over the ethernet interface.