Renamed thread_enabled flag to thread_enabled_platform

thread_enabled flag is used in Connectivity module.
The module and the platform are not built together, so using the same
flag for both may lead the flag value mismatches.

This CL also adds thread_enabled_platform to flags.aconfig file

Test: TH
Change-Id: I1be418e807c77c456cdcfa99824d3d37b00edcd6