Replace the custom $bootcmd with that from <config_distro_bootcmd.h>.
There should be no functional change, since the new generic $bootcmd was
derived strongly from tegra-common-post.h, after which this part of
rpi_b.h was modelled.

The #defines to enable/disable U-Boot commands/features were moved
earlier in rpi_b.h, so they are set up before config_distro_bootcmd.h
is included, since it tests whether certain features should be included
based on those defines.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Simon Glass <sjg@chromium.org>