Update change log and spec for wrt-plugins-tizen_0.2.80

Update change log and spec for wrt-plugins-tizen_0.2.80

Changed Modules : SystemInfo

[Version] 0.2.80
[Project] GT-I8800, Public
[Title] SEL Verification
[Team] WebAPI
[BinType] PDA
[Customer] Open

[Issue#] N/A
[Problem] Cellularnetwork status attribute's mean is wrong.
[Cause] change getPropertyValue API about CellularNetwork
[Solution] Chanage spec & Chanage code

[Issue#] WEB-1850
[Problem] if property attribute's value is null, occured crash
[Cause] getpropertyvalue api crash
[Solution] added protect code

[SCMRequest] N/A

Change-Id: I51a90223214e9e15a719714286d13fdf7b02cf36