MFC r258786, r258796, r258797, r258847, r258853, r258949:

MFC r258786, r258796, r258797, r258847, r258853, r258949:

r258786:
  Move OPTIONS_UNSET outside of the PBUILD_FLAGS variable, otherwise
  the textproc/docproj port build fails.

r258796 (hrs):
  - Prevent TARGET and TARGET_ARCH from being defined as empty when
    TARGET="" and/or TARGET_ARCH="" is specified.
  - Remove extra ().

r258797 (hrs):
  Use standard CLEANFILES/CLEANDIRS and clean target in bsd.obj.mk.

r258847:
  Provide reproducibility between builds by building pkg(8) from ports,
  instead of using pkg-bootstrap.

  This should resolve a problem that was discovered during 10.0-BETA4
  freebsd-update(8) builds,

r258853:
  Turn off the dvdrom target by default.  dvd1.iso is now built by
  specifying 'WITH_DVD=1' during 'make release'.

  This caused some problems during the freebsd-update builds for
  10.0-BETA4.

r258949:
  Ensure WITH_DVD is not empty to prevent 'WITH_DVD= ' from evaluating
  to true.

Approved by:	re (hrs)
Sponsored by:	The FreeBSD Foundation

git-svn-id: svn://svn.freebsd.org/base/stable/10@258952 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f