You can use the standard Raspberry Pi image and simply download and run the program as follows:
// Downloads the firmware code
git clone https://github.com/OpenSprinkler/OpenSprinkler-Firmware
// Enter the directory that contains the above code
cd OpenSprinkler-Firmware
// Compile the firmware (this step will also ask if you want to run on system startup)
./build.sh ospi
// Run the program now (if you installed on system startup you could also start that way)
./OpenSprinkler
Hope that helps!
Update: You can even run the code on OS X, etc by using ‘build.sh demo’. The demo is also publicly available here: http://demo.opensprinkler.com.