RemoteLinux: Change data type for better extensibility.

Other plugins are expected to build on the RemoteLinux infrastructure
in the future, providing OS types of their own. A string-based
identifier is much better suited for that than the currently used
enum approach.

Change-Id: I702a1ed7628d908d9c68ae0910ad1a6f1b92786f
Reviewed-on: http://codereview.qt.nokia.com/357


Reviewed-by: Tobias Hunger <tobias.hunger@nokia.com>