UBUNTU: [Packaging] final-checks: Depend on makefile flavours

UBUNTU: [Packaging] final-checks: Depend on makefile flavours

The available ABI files do not depend on the config flavours but
those actually built. So instead of looking at the config files,
parse the flavours variable of the <arch>.mk files.

This handles a special case which was found for the current
xenial/hwe tree. This is now based on bionic which builds a
snapdragon flavour for arm64. However that flavour is not done
in xenial because it is a separate tree there.

While the flavour gets removed from the .mk file, we rather would
like to preserve the snapdragon.flavour.config file because removing
it will cause re-arrangement of config options which would happen
everytime we update from the bionic master.

However the current final-checks would then fail because of course
there is no build and therefor no ABI files for that flavour.

Ignore: yes
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>