So 'ro' is eiaser to do when you cut /var/ out of the main partiion...
grab a 4gb microsd (or larger) and do:
tar xf debian-8.1-console-armhf-2015-07-26.tar.xz
cd debian-8.1-console-armhf-2015-07-26/
sudo ./setup_sdcard.sh --mmc /dev/sdX --dtb beaglebone --ro
# the "--ro" sets things up for read-only...
On first bootup "/" is rw, let it generate the files on initial bootup
and setup your ethernet, then open:
/etc/fstab
you'll notice a couple comments about "ro" switch from the "rw" line
to "ro" and reboot..
Regards,