Inline usages of LOCAL_STEM

Having no variables in include/inherit-product statements
makes the conversion to starlark cleaner.

Bug: 211659921
Test: Presubmits
Change-Id: I9253547c1562b9a781a82ddf448dca1cf0bd8d88
diff --git a/self-extractors_hikey960/root/device-vendor.mk b/self-extractors_hikey960/root/device-vendor.mk
index 31f5c3a..f4fb1bb 100644
--- a/self-extractors_hikey960/root/device-vendor.mk
+++ b/self-extractors_hikey960/root/device-vendor.mk
@@ -14,6 +14,4 @@
 # limitations under the License.
 #
 
-LOCAL_STEM := device-partial.mk
-
-$(call inherit-product-if-exists, vendor/linaro/hikey960/hisilicon/$(LOCAL_STEM))
+$(call inherit-product-if-exists, vendor/linaro/hikey960/hisilicon/device-partial.mk)