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

README.md

@@ -419,7 +419,8 @@ soong_config_module_type {
     name: "acme_cc_defaults",
     module_type: "cc_defaults",
     config_namespace: "acme",
-    variables: ["board", "feature"],
+    variables: ["board"],
+    bool_variables: ["feature"],
     properties: ["cflags", "srcs"],
 }
 
@@ -427,10 +428,6 @@ soong_config_string_variable {
     name: "board",
     values: ["soc_a", "soc_b"],
 }
-
-soong_config_bool_variable {
-    name: "feature",
-}
 ```
 
 This example describes a new `acme_cc_defaults` module type that extends the