Bug 516330 - Make terminal.connector.serial REQUIRE gnu.io non-optional

As the o.e.tm.terminal.connector.serial , which is based on RXTX,
was taken out of the master terminal.feature, it is now only available
as a separate feature. Consequently, it only makes sense installing
when RXTX is also available - the RXTX dependency was thus made
non-optional.

Change-Id: If6528a6518da57335c7449def359e0f645c77b67
Signed-off-by: Martin Oberhuber <mober.at+eclipse@gmail.com>