I think v1.8 has different board definition file format. Basically, this is referring to step 4 of the built instructions for 2.x:
https://openthings.freshdesk.com/support/solutions/articles/5000165132-how-to-compile-opensprinkler-firmware
where you have downloaded the aopensprinkler.zip board profiles. You most likely need to open the boards.txt and add the missing build.board preferenc as the error message implies. You can take a look at the default arduino boards.txt to see examples of what build.board refers to.