qcacld-3.0: Use makefile flag to select DLKM makefile

The kernel is transitioning from AndroidKernelModule.mk to
Build_external_kernelmodule.mk as the helper makefile for building
DLKMs. As the new makefile was introduced for the "taro" platform,
initially a platform check was introduced to determine which makefile
to include.  However this is not a scalable approach.

Recently the flag TARGET_USES_KERNEL_PLATFORM was added to the build
system, so update the logic such that when this flag is set to 'true'
we will use Build_external_kernelmodule.mk.

Change-Id: I2200b27753263b931cebdbff5445d50b1fc2b94f
CRs-Fixed: 2913254

Android.mk

@@ -193,7 +193,8 @@ $(call wlog,"generate soft link because GENERIC_ODM_IMAGE not true")
 endif
 endif
 
-ifeq ($(TARGET_PRODUCT), taro)
+$(call wlog,TARGET_USES_KERNEL_PLATFORM=$(TARGET_USES_KERNEL_PLATFORM))
+ifeq ($(TARGET_USES_KERNEL_PLATFORM),true)
     include $(DLKM_DIR)/Build_external_kernelmodule.mk
 else
     include $(DLKM_DIR)/AndroidKernelModule.mk