sm6250-common: add root makefile

Android.mk

+#
+# Copyright (C) 2020 The LineageOS Project
+#
+# SPDX-License-Identifier: Apache-2.0
+#
+
+LOCAL_PATH := $(call my-dir)
+
+ifneq ($(filter placeholder,$(TARGET_DEVICE)),)
+include $(call all-makefiles-under,$(LOCAL_PATH))
+endif