distro_distro_bootcmd: use CONFIG_BOOTCOMMAND instead of setting bootcmd=

distro_distro_bootcmd: use CONFIG_BOOTCOMMAND instead of setting bootcmd=

Move the bootcmd commands into a seperate distro_bootcmd environment
variable. Allowing a user to easily launch the distro boot sequence if
the default bootcmd did not default to distro boot commands.

Also set CONFIG_BOOTCOMMAND to "run distro_bootcmd" if it hasn't been
configured yet rather then putting it directly in the environment. This
allows boards to make the distro boot commands available without
necessarily default to them or to use them as a fallback after running
some board specific commands instead.

Signed-off-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>
Reviewed-by: Stephen Warren <swarren@nvidia.com>