debian/control.in/main: build-depends on debhelper (>= 12.5) to get automatic udeb support.

* debian/control.in/main: build-depends on debhelper (>= 12.5) to get
  automatic udeb support.
* debian/rules.d/debhelper.mk: build the udeb packages first, drop call to
  debian/shlibs-add-udebs.
* debian/shlibs-add-udebs: removed, not used anymorre.