[IOT-2628][EasySetup]Update WifiConf prop. types

[IOT-2628][EasySetup]Update WifiConf prop. types

- Change properties from integer to string enums: swmt, swf, wat, wet
- Change supported frequencies from single value to array.
- Create type converters for enum to string and string to enum.
- Remove EOF marker in enum WIFI_MODE.
- Update Enrollee sample apps (Linux and Tizen).
- Cleanup some unused code in easysetup.c.
- Fixed unit test cases for Enrollee and Mediator.

Note: This impacts Easy Setup Enrollee API "ESSetDeviceProperty".

Thanks-To: Abhishek Pandey <abhi.siso@samsung.com>
Change-Id: Iea8191616cea89a0273e0f5486bd259735e914fa
Signed-off-by: Abhishek Pandey <abhi.siso@samsung.com>
Signed-off-by: Ankur Bansal <ankur.b1@samsung.com>