deprecated handling: introduce BR2_DEPRECATED_SINCE_xxxx_xx

deprecated handling: introduce BR2_DEPRECATED_SINCE_xxxx_xx

In order to keep better track of when a feature got deprecated, and hence
when it can be removed, a new set of symbols BR2_DEPRECATED_SINCE_xxxx_xx is
introduced. These symbols are automatically selected when BR2_DEPRECATED is
selected, and thus are transparent to the user.
A deprecated feature will no longer depend on BR2_DEPRECATED directly, but
rather on the appropriate BR2_DEPRECATED_SINCE_xxxx_xx. If that symbol does
not yet exist, it has to be created in Config.in.
When removing a deprecated feature, one should also check whether this was
the last feature using the BR2_DEPRECATED_SINCE_xxxx_xx symbol, in which
case the latter can be removed from Config.in.

A followup patch will make sure the overview is added to the list of
deprecated features in the manual, so that a buildroot core developer can
easily determine which features to remove in a given development cycle.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>