nuttx only show stderr

-failures will still be displayed, but we don't need to see normal
successful compile output

cmake/nuttx/px4_impl_nuttx.cmake

@@ -229,7 +229,7 @@ function(px4_nuttx_add_export)
 		COMMAND ${CP} -r ${CMAKE_SOURCE_DIR}/nuttx-configs/${CONFIG} ${nuttx_src}/nuttx/configs
 		COMMAND cd ${nuttx_src}/nuttx/tools && ./configure.sh ${CONFIG}/nsh
 		COMMAND ${ECHO} Exporting NuttX for ${CONFIG}
-		COMMAND ${MAKE} --no-print-directory --quiet -C ${nuttx_src}/nuttx -j${THREADS} -r CONFIG_ARCH_BOARD=${CONFIG} export
+		COMMAND ${MAKE} --no-print-directory --quiet -C ${nuttx_src}/nuttx -j${THREADS} -r CONFIG_ARCH_BOARD=${CONFIG} export > /dev/null
 		COMMAND ${CP} -r ${nuttx_src}/nuttx/nuttx-export.zip ${CMAKE_BINARY_DIR}/${CONFIG}.export
 		DEPENDS ${config_files} ${DEPENDS} __nuttx_copy_${CONFIG})