update-alternatives.bbclass: fix old override syntax

Function 'gen_updatealternativesvardeps' still used old override
syntax when fetching variable flags. Update to use ':' instead to match
recipe meta data. This was found by review and no real issue encountered
but it is a bug that affects variable dependencies and can affect rebuilds
as task hashes might not be accurate.

Signed-off-by: Peter Bergin <peter.bergin@windriver.com>
Signed-off-by: Peter Bergin <peter@berginkonsult.se>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>