board/BuR/common: simplify access to devicetree

instead of polling everytime the environment, we take usage of the global
gd->fdt_blob variable and check it only against NULL.

Variable "dtbaddr" from environment is needed only one time on loading the
devicetree within "load_devicetree()"

Signed-off-by: Hannes Petermaier <oe5hpm@oevsv.at>