Toolchain: Add type() method.

Rename typeName() to typeDisplayName() to make it clear that this is
a user visible string that is translated. Add type() method to provide
a simple string that can be used by build system plugins to find out
which kind of toolchain currently is in use.

Change-Id: I7c3f050aa82c8b17071fd03ee142c5ae2f17b1de
Reviewed-by: Daniel Teske <daniel.teske@nokia.com>
Reviewed-by: Marco Bubke <marco.bubke@nokia.com>