Make sure you have the android-tools-adb, android-tools-fsutils and
android-tools-fastboot packages installed before running the script.

To install an ubuntu system-image to a device, you need a cdimage rootfs
tarball for ubuntu touch devices (you can create the tarball with the
rootstock-touch tool shipped along with the rootstock-touch-installer
script or you can directly download one from:

http://cdimage.ubuntu.com/ubuntu-touch/daily-preinstalled/current/trusty-preinstalled-touch-armhf.tar.gz

Additionally to the ubuntu rootfs you need android system.img, boot.img
and recovery.img which will be used for the lxc container that provides
the hardware abstraction layer used by ubuntu touch. These img files are
usually created by building the android tree for your device based on
the git tree from
http://phablet.ubuntu.com (please see the porting guide at
https://wiki.ubuntu.com/Touch/Porting for details on creating andorid
images for your device.)

To prepare your device you now need to put it into bootloader mode.
First flash the boot.img and recovery.img files with fastboot:

fastboot flash recovery /path/to/recovery.img
fastboot flash boot /path/to/boot.img

Now select "Boot to Recovery" (or similar) in the bootloader menu

Once the device booted into recovery mode run rootsotck-touch-install:

./rootsotck-touch-install ./trusty-trusty-preinstalled-touch-armhf.tar.gz ./system.img

(the above example assumes the needed files are in the current dir)

rootstock-touch-install knows an option (-k) to keep the user data
(/home/phablet/) untouched, in case you only want to install a system
upgrade.

Note that by default everything gets wiped !.