[bsp][stm32] add new bsp ci

.travis.yml

@@ -90,6 +90,7 @@ env:
   - RTT_BSP='stm32/stm32f429-armfly-v6' RTT_TOOL_CHAIN='sourcery-arm'
   - RTT_BSP='stm32/stm32f429-atk-apollo' RTT_TOOL_CHAIN='sourcery-arm'
   - RTT_BSP='stm32/stm32f429-fire-challenger' RTT_TOOL_CHAIN='sourcery-arm'
+  - RTT_BSP='stm32/stm32f446-st-nucleo' RTT_TOOL_CHAIN='sourcery-arm'
   - RTT_BSP='stm32/stm32f767-atk-apollo' RTT_TOOL_CHAIN='sourcery-arm'
   - RTT_BSP='stm32/stm32f767-fire-challenger' RTT_TOOL_CHAIN='sourcery-arm'
   - RTT_BSP='stm32/stm32l475-atk-pandora' RTT_TOOL_CHAIN='sourcery-arm'