[build] Add frdm-k64f into travis-ci building

.travis.yml

@@ -62,5 +62,5 @@ env:
   - RTT_BSP='lpc408x' RTT_TOOL_CHAIN='sourcery-arm'
   - RTT_BSP='beaglebone' RTT_TOOL_CHAIN='sourcery-arm'
   - RTT_BSP='zynq7000' RTT_TOOL_CHAIN='sourcery-arm'
-  - RTT_BSP='frdm-k64f' RTT_ROOT_CHAIN='sourcery-arm'
+  - RTT_BSP='frdm-k64f' RTT_TOOL_CHAIN='sourcery-arm'