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/BoardConfigVendor.mk b/self-extractors_hikey960/root/BoardConfigVendor.mk
index 1517386..e70a67d 100644
--- a/self-extractors_hikey960/root/BoardConfigVendor.mk
+++ b/self-extractors_hikey960/root/BoardConfigVendor.mk
@@ -12,6 +12,4 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-LOCAL_STEM := hikey960/BoardConfigPartial.mk
-
--include vendor/linaro/$(LOCAL_STEM)
+-include vendor/linaro/hikey960/BoardConfigPartial.mk