import generate-release.sh from CURRENT
use our ports tree

- supoport relative path to CHROOTDIR
- if CHROOTDIR does not exist, create it
pass MAKEOBJDIRPREFIX to make it possible to use different OBJDIR
use generate-release.sh

- use OBJDIR under ${.CURDIR}. the first buildworld does not use
  chroot(8)
- support cross-build (TARGET=i386 TARGET_ARCH=i386)
- drop multiple builds support in batch (i.e. build 9.0 and 9.1), which
  complicates the script
- drop -STABLE and CURRENT support. you can build non-RELEASE by
  invoking generate-release.sh

there are missing futures, see TODO
Merge pull request #1 from trombik/generate_release

use generate-release.sh from CURRENT