sdk: add DLL_EXTENSION as an atree variable

This patch defines the DLL_EXTENSION variable to the host
shared library suffix (i.e. either ".so", ".dll" or ".dylib")
when packaging the SDK.

This is required by another patch to sdk/build/tools.atree that
will use ${DLL_EXTENSION} to copy the GLES emulation host libraries
to $SDK/tools/lib

Change-Id: I120d39e905afb453b0702d5f0eb2707f214385cc

core/Makefile

@@ -1409,6 +1409,7 @@ $(INTERNAL_SDK_TARGET): $(deps)
 			-v "OUT_DIR=$(OUT_DIR)" \
 			-v "TARGET_ARCH=$(TARGET_ARCH)" \
 			-v "TARGET_CPU_ABI=$(TARGET_CPU_ABI)" \
+			-v "DLL_EXTENSION=$(HOST_SHLIB_SUFFIX)" \
 			-o $(PRIVATE_DIR) && \
 		cp -f $(target_notice_file_txt) \
 				$(PRIVATE_DIR)/platforms/android-$(PLATFORM_VERSION)/images/$(TARGET_CPU_ABI)/NOTICE.txt && \