Implement LOCAL_TARGET_REQUIRED_MODULES

to allow a host module to specify LOCAL_TARGET_REQUIRED_MODULES
to indicate that the device module should be built.

Bug: b/64580523
Test: add "LOCAL_TARGET_REQUIRED_MODULES := \
      CtsDeviceOsTestApp CtsHostProcfsTestApp" to \
      cts/hostsidetests/os/Android.mk, run \
      `m -j CtsOsHostSideTestCases`, check \
      that CtsDeviceOsTestApp.apk was built

Change-Id: I1457a51f7cd922c059aa62955d8693b1a7539fd9
diff --git a/core/clear_vars.mk b/core/clear_vars.mk
index 1fba952..c38059c 100644
--- a/core/clear_vars.mk
+++ b/core/clear_vars.mk
@@ -233,6 +233,7 @@
 LOCAL_STATIC_LIBRARIES:=
 LOCAL_STRIP_MODULE:=
 LOCAL_SYSTEM_SHARED_LIBRARIES:=none
+LOCAL_TARGET_REQUIRED_MODULES:=
 LOCAL_TEST_DATA:=
 LOCAL_TEST_MODULE_TO_PROGUARD_WITH:=
 LOCAL_TIDY:=