configure: move deprecated feature processing to supported_target

This is the common point at which we validate targets so it makes
sense to add_to deprecated_features here. It will make future target
deprecation easier as we only need to tweak one list.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20200915134317.11110-4-alex.bennee@linaro.org>