board/BuR: invalidate ${dtbaddr} before cfgscr

The first memory location of ${dtbaddr} may be still valid after a warm
restart of the machine and 'fdt addr ${dtbaddr}' doesn't recognize that
the cfgscript didn't run properly and fallback mechanism with copying
the internal fdt ${fdtcontroladdr} to ${dtbaddr} doesn't catch this.

To get sure that we have proper failsafe behaviour we simply zero the
first memory location of ${dtbaddr} for getting sure that the fdt is
invalid if cfgscript didn't run.

Signed-off-by: Hannes Schmelzer <hannes.schmelzer@br-automation.com>

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