soong_config: bool_variables shortcut

Using a lot of boolean variables can become very verbose without adding
really any new information:

      variables: ["a", "b", "c"],
  }

  soong_config_bool_variable {
      name: "a",
  }

  soong_config_bool_variable {
      name: "b",
  }

  soong_config_bool_variable {
      name: "c",
  }

Now turns into:

      bool_variables: ["a", "b", "c"],
  }

Test: built-in tests
Change-Id: If5455a38433431c7ecbce1e5b32cfbb47f42602a

android/soong_config_modules_test.go

@@ -43,7 +43,8 @@ func TestSoongConfigModule(t *testing.T) {
 			name: "acme_test_defaults",
 			module_type: "test_defaults",
 			config_namespace: "acme",
-			variables: ["board", "feature1", "feature2", "FEATURE3"],
+			variables: ["board", "feature1", "FEATURE3"],
+			bool_variables: ["feature2"],
 			properties: ["cflags", "srcs"],
 		}
 
@@ -56,10 +57,6 @@ func TestSoongConfigModule(t *testing.T) {
 			name: "feature1",
 		}
 
-		soong_config_bool_variable {
-			name: "feature2",
-		}
-
 		soong_config_bool_variable {
 			name: "FEATURE3",
 		}