build - allow ABIVERS to be overridden in Makefile.usr

* Allow ABIVERS to be overridden in Makefile.usr.  (Defaults to uname -P).
netgraph7: Fix a bug in the Makefile.

* The ng_mppc node should not be built if the NO_CRYPT make
  variable is set.
drm: Improve the BUG() macro