update-alternatives: Simplfy variable dependency logic

update-alternatives: Simplfy variable dependency logic

When looking at bitbake parsing speed issues, I noticed a lot of weird looking
variables from the update-alternatives class. It is possible this was written
before variable dependencies could handle flags. It can handle flags now so
simplfy the code to take advantage of that and avoid the indirection variables.

The win here is a significant reduction in the number of variables, which
in turn significantly reduces the looping bitbake's taskhash calculation code
needs to do.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>