Use "*" to indicate default capabilities in config

Instead of using an empty string, make the use of default capabilities
explicit by requiring "*" to be specified.

This behavior is more consistent and allows OEMs to explicitly configure
an interface without any capabilities (apart from those which are
hard-coded). (Technically, they could do so by specifying one of the
hardcoded capabilities in the config).

Test: TH
Change-Id: I1d25e5c4b302b0dbde5e1bb91fc1ed5a7b08be9d
3 files changed