Fix atmconfig compilation when MK_ATM == yes and MK_BSNMP == no

Makefile.inc1:
Always compile gensnmptree with bootstrap-tools when MK_BSNMP != no
instead of depending on a potentially stale tool installed on the build host

sbin/atm/atmconfig/Makefile:
- Always remove oid.h to avoid cluttering up the build/src tree.
- Consolidate all of the RESCUE/MK_BSNMP != no logic under one
conditional to improve readability
- Remove unnecessary ${.OBJDIR} prefixing for oid.h and use ${.TARGET} instead
  of spelling out oid.h
- Add a missing DPADD for ${LIBCRYPTO} when compiled MK_BSNMP == yes and
  MK_OPENSSL == yes and not compiling for /rescue/rescue

sbin/atm/atmconfig/main.c:
Change #ifndef RESCUE to #ifdef WITH_BSNMP in main.c to make it
clear that we're compiling bsnmp support into atmconfig

Approved by: jmmv (mentor)
Phabric: D579
PR: 143830
MFC after: 2 weeks
Sponsored by: EMC / Isilon Storage Division
Make the USB and ZFS devd configuration files optional depending on the
values of MK_USB/MK_ZFS

Making zfs.conf optional resolves PR # 186971

PR: 186971
Phabric: D606
Approved by: jmmv (mentor)
Sponsored by: EMC / Isilon Storage Division