mk: filter duplicate configuration entries

mk: filter duplicate configuration entries

Due to the hierarchy and the demand to keep the base config showing all
options, some config keys end up multiple times in the .config file.

Due to the way the actual config is sourced only the last entry is
important. That can confuse people changing values in .config which
are then ignored.

A suggested solution was to filter for duplicates at the end of the
actual config step which is implemented here.

Signed-off-by: Christian Ehrhardt <christian.ehrhardt@canonical.com>