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/soongconfig/modules.go

@@ -109,6 +109,9 @@ type ModuleTypeProperties struct {
 	// the list of SOONG_CONFIG variables that this module type will read
 	Variables []string
 
+	// the list of boolean SOONG_CONFIG variables that this module type will read
+	Bool_variables []string
+
 	// the list of properties that this module type will extend.
 	Properties []string
 }
@@ -146,6 +149,18 @@ func processModuleTypeDef(v *SoongConfigDefinition, def *parser.Module) (errs []
 	}
 	v.ModuleTypes[props.Name] = mt
 
+	for _, name := range props.Bool_variables {
+		if name == "" {
+			return []error{fmt.Errorf("bool_variable name must not be blank")}
+		}
+
+		mt.Variables = append(mt.Variables, &boolVariable{
+			baseVariable: baseVariable{
+				variable: name,
+			},
+		})
+	}
+
 	return nil
 }