Add tensorflow downloads folder to BYPRODUCTS

Add tensorflow download location to BYPRODUCTS for add_custom_target to
make cmake aware of the generated files. This makes 'make clean' to wipe
the folder.

Change-Id: I81535ccf0ecb8d948990fc4ec214c09213307b07
diff --git a/tensorflow.cmake b/tensorflow.cmake
index 27a8918..7e02636 100644
--- a/tensorflow.cmake
+++ b/tensorflow.cmake
@@ -58,6 +58,7 @@
 # Command and target
 add_custom_target(tflu_gen ALL
                   COMMAND make -j${J} -f ${TFLU_PATH}/tools/make/Makefile microlite TARGET=${TFLU_TARGET} TARGET_ARCH=${TFLU_TARGET_ARCH} CC_TOOL=${TFLU_CC} CXX_TOOL=${TFLU_CXX} AR_TOOL=${TFLU_AR} GENDIR=${TFLU_GENDIR} CMSIS_PATH=${CMSIS_PATH} ETHOSU_DRIVER_PATH=${CORE_DRIVER_PATH} ETHOSU_DRIVER_LIBS=${TFLU_ETHOSU_LIBS} TAGS="${TFLU_TAGS}" BUILD_TYPE=${TFLU_BUILD_TYPE}  $<$<BOOL:${TFLU_OPTIMIZATION_LEVEL}>:OPTIMIZATION_LEVEL=${TFLU_OPTIMIZATION_LEVEL}>
+                  BYPRODUCTS ${CMAKE_CURRENT_SOURCE_DIR}/tensorflow/tensorflow/lite/micro/tools/make/downloads
                   WORKING_DIRECTORY ${TENSORFLOW_PATH})
 
 # Create library and link library to custom target