MFC r273653:
  Fix a few issues with creating VOLUME_LABEL for the
  installation ISOs:

   - TYPE, BRANCH, and REVISION are only defined if
     OSRELEASE is not defined, so in situations where
     one might set OSRELEASE for an in-house ISO build,
     VOLUME_LABEL would be empty.

   - makefs(8) limits the volume label to 32 characters,
     which for the powerpc64 case, OSRELEASE expands to
     FreeBSD-11.0-CURRENT-powerpc-powerpc64.  Even with
     removing the prefixing 'FreeBSD-', the string is 30
     characters long, leaving zero room for suffixing the
     type of ISO media (BO for bootonly, CD for cdrom, and
     DVD for dvdrom).

  Resolve these by defining VOLUME_LABEL when defining
  OSRELEASE if unset.  If OSRELEASE is defined by the
  builder, use the OSRELEASE from that definition as the
  VOLUME_LABEL.

  In addition, for cases where both TARGET and TARGET_ARCH
  are used for the VOLUME_LABEL, use TARGET_ARCH if it
  differs from TARGET.

  There are probably a few sharp edges here yet, but these
  problems are going to affect the powerpc/powerpc64 builds
  for 10.1-RELEASE, so the immediate concern is fixing the
  underlying problem at hand quickly, and less so about the
  elegance of the fix.

Insta-MFC ok:	re (kib)
Sponsored by:	The FreeBSD Foundation