bitbake: toaster: stop modifying OEROOT in toaster script

bitbake: toaster: stop modifying OEROOT in toaster script

Setting OEROOT in toaster script makes oe-init-build-env to
break with error:
  bash: ../bitbake/bin/../../scripts/oe-buildenv-internal: No such file or directory
This happens because OEROOT contains path relative to build
directory.

Renamed OEROOT to OE_ROOT and unset it after it's used.

(Bitbake rev: 3b0967b160dc6123ef75e6f378221347bd923f1b)

Signed-off-by: Ed Bartosh <ed.bartosh@linux.intel.com>
Signed-off-by: Michael Wood <michael.g.wood@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>