rtl8721csm: Improve download script
rtl8721csm: Improve download script

- Remove duplicated write operation for kernel partitions
- Unify download functions for specific partition
- Remove hard-coded and clean codes

jsdosa/TizenRT,sunghan-chang/TizenRT,pillip8282/TizenRT,junmin-kim/TizenRT,Samsung/TizenRT,jsdosa/TizenRT,sunghan-chang/TizenRT,jeongarmy/TizenRT,jsdosa/TizenRT,junmin-kim/TizenRT,pillip8282/TizenRT,sunghan-chang/TizenRT,an4967/TizenRT,pillip8282/TizenRT,Samsung/TizenRT,sunghan-chang/TizenRT,Samsung/TizenRT,jeongarmy/TizenRT,jeongchanKim/TizenRT,pillip8282/TizenRT,junmin-kim/TizenRT,jeongarmy/TizenRT,junmin-kim/TizenRT,junmin-kim/TizenRT,sunghan-chang/TizenRT,jeongarmy/TizenRT,jeongarmy/TizenRT,sunghan-chang/TizenRT,junmin-kim/TizenRT,jsdosa/TizenRT,Samsung/TizenRT,Samsung/TizenRT,jeongchanKim/TizenRT,jeongchanKim/TizenRT,pillip8282/TizenRT,sunghan-chang/TizenRT,an4967/TizenRT,jsdosa/TizenRT,jeongchanKim/TizenRT,jeongarmy/TizenRT,pillip8282/TizenRT,Samsung/TizenRT,an4967/TizenRT,Samsung/TizenRT,jsdosa/TizenRT,junmin-kim/TizenRT,pillip8282/TizenRT,jeongchanKim/TizenRT,jeongarmy/TizenRT,jsdosa/TizenRT,an4967/TizenRT,jeongchanKim/TizenRT,an4967/TizenRT,jeongchanKim/TizenRT,an4967/TizenRT,an4967/TizenRT