board/BuR/common: incorrect check of dtb

The logical expression to check the dtb is incorrect in
load_devicetree.

The problem was indicated by cppcheck.

The inconsistent variable name dtppart is changed to dtbpart.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Hannes Schmelzer <oe5hpm@oevsv.at>
Acked-by: Hannes Schmelzer <oe5hpm@oevsv.at>