bitbake: toaster: use OneToOneField instead of ForeignKey

bitbake: toaster: use OneToOneField instead of ForeignKey

Used OneToOneField to reference BuildRequest in BRBitbake model.

Fixed django warning:
WARNINGS: Setting unique=True on a ForeignKey has the same effect
          as using a OneToOneField.

(Bitbake rev: aaa4319ebbb06facb77b4ba936cf3aa2068ff238)

Signed-off-by: Elliot Smith <elliot.smith@intel.com>
Signed-off-by: Ed Bartosh <ed.bartosh@linux.intel.com>
Signed-off-by: brian avery <avery.brian@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>