tbs2910: Disable VxWorks image booting support

There are currently no known users of this functionality on this
platform, disable it to prepare for additional VxWorks functionality
that would cause this platform to fail to link.

Cc: Soeren Moch <smoch@web.de>
Signed-off-by: Tom Rini <trini@konsulko.com>
Acked-by: Soeren Moch <smoch@web.de>