Document config_distro_bootcmd environment variables for interactive booting.

config_distro_bootcmd.h defines a common boot environment for multiple
platforms, including several environment variables that are intended for
interactive use by an end-user.  Document which variables are considered
public interfaces that must remain compatible in future u-boot versions.

Signed-off-by: Karsten Merker <merker@debian.org>
Acked-by: Stephen Warren <swarren@nvidia.com>