debianpkg: normalize rules files

debianpkg: normalize rules files

downstream packagers prefer to change defaults in a trackable way by
changing the rules file directly, rather than setting environment
variables.  Use '?=' (set if not already set in the environment or on
the make cmdline) to set defaults rather than alternating between
ifeq/ifneq clauses, which is harder to follow and edit.  Change any
existing `ifneq(...,0)' cases to `ifeq(...,1)', and get rid of any
ifdef/ifndefs, as ?= guarantees he value will be defined in one way or
another.  This allows the old behavior of overriding via the
environment while simplifying the workflow for anyone editing or
extending the current logic and defaults.

Portability note about '?=': it is GNU-make specific, but so is
ifeq/ifneq, which is/was used in this file, and this file is specific
to debian-based system as it is, so I don't consider it to be a
problem in this case.

Added any missing defaults (WANT_SNMP, WANT_CUMULUS_NODE) and
made it so that USE_XXX is always set for the sake of consistency.
Also brought a few changes from base debianpkg/rules into the
backports versions of the files where they were missing.

Signed-off-by: Silas McCroskey <smccroskey@cumulusnetworks.com>