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
diff --git a/android/soong_config_modules.go b/android/soong_config_modules.go
index 198108d..fa1e204 100644
--- a/android/soong_config_modules.go
+++ b/android/soong_config_modules.go
@@ -88,7 +88,8 @@
 //         name: "acme_cc_defaults",
 //         module_type: "cc_defaults",
 //         config_namespace: "acme",
-//         variables: ["board", "feature"],
+//         variables: ["board"],
+//         bool_variables: ["feature"],
 //         properties: ["cflags", "srcs"],
 //     }
 //
@@ -97,10 +98,6 @@
 //         values: ["soc_a", "soc_b"],
 //     }
 //
-//     soong_config_bool_variable {
-//         name: "feature",
-//     }
-//
 // If an acme BoardConfig.mk file contained:
 //
 //     SOONG_CONFIG_NAMESPACES += acme
@@ -149,7 +146,8 @@
 //         name: "acme_cc_defaults",
 //         module_type: "cc_defaults",
 //         config_namespace: "acme",
-//         variables: ["board", "feature"],
+//         variables: ["board"],
+//         bool_variables: ["feature"],
 //         properties: ["cflags", "srcs"],
 //     }
 //
@@ -158,10 +156,6 @@
 //         values: ["soc_a", "soc_b"],
 //     }
 //
-//     soong_config_bool_variable {
-//         name: "feature",
-//     }
-//
 //     acme_cc_defaults {
 //         name: "acme_defaults",
 //         cflags: ["-DGENERIC"],
diff --git a/android/soong_config_modules_test.go b/android/soong_config_modules_test.go
index 6ad88a2..1cf060d 100644
--- a/android/soong_config_modules_test.go
+++ b/android/soong_config_modules_test.go
@@ -43,7 +43,8 @@
 			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"],
 		}
 
@@ -57,10 +58,6 @@
 		}
 
 		soong_config_bool_variable {
-			name: "feature2",
-		}
-
-		soong_config_bool_variable {
 			name: "FEATURE3",
 		}
 	`
diff --git a/android/soongconfig/modules.go b/android/soongconfig/modules.go
index aa4f5c5..2d6063d 100644
--- a/android/soongconfig/modules.go
+++ b/android/soongconfig/modules.go
@@ -109,6 +109,9 @@
 	// 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 @@
 	}
 	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
 }