add an option to exclude the TWRP app

* Allow device maintainer to exclude the twrp app and disable
   the installation prompt. Mostly the app is useless for
   every unofficially supported devices.

 * BoardConfig flag: 'TW_EXCLUDE_TWRPAPP := true'

Original commit: https://gerrit.omnirom.org/#/c/android_bootable_recovery/+/27694/

Change-Id: I0aa1fb2ebe03ceb19c766178544039a82497cdd4

data.cpp

@@ -934,11 +934,15 @@ void DataManager::SetDefaultValues()
 	mConst.SetValue("tw_app_installed_in_system", "0");
 #else
 	mConst.SetValue("tw_oem_build", "0");
+#ifdef TW_EXCLUDE_TWRPAPP
+	mConst.SetValue("tw_app_prompt", "-1");
+#else
 	mPersist.SetValue("tw_app_prompt", "1");
 	mPersist.SetValue("tw_app_install_system", "1");
 	mData.SetValue("tw_app_install_status", "0"); // 0 = no status, 1 = not installed, 2 = already installed
 	mData.SetValue("tw_app_installed_in_system", "0");
 #endif
+#endif
 #ifndef TW_EXCLUDE_NANO
 	mConst.SetValue("tw_include_nano", "1");
 #else