The Zoning Board of Appeal on Tuesday approved plans for a four-story, 21-unit live/work condo building at 2 Hillsboro St. in Dorchester - with 18 of the units aimed at artists. The units, which will be split between studios and one- and two-bedroom units, with one three-bedroom unit, will all be sold to people making between 80% and 100% of the Boston area median income. The