Add support for returning optional values

Some commands have optional return values. This allows the handlers to
be defined as returning an optional<T> value and then in the body of the
handler set the value in the optional or not.

Tested-by: unit test runs successfully

Change-Id: Ib38a4589609fb1eb192106e511c9ee3a507ac42f
Signed-off-by: Vernon Mauery <vernon.mauery@linux.intel.com>
2 files changed