add README.distro file

Add documentation on how to setup a system to use the generic distro
configs and boot commands. This spells out what is needed to make a
system conformant, but does not limit the board to only the defaults.

Signed-off-by: Dennis Gilmore <dennis@ausil.us>
[swarren, added concept, user config, BOOT_TARGET_DEVICES sections.
edited the rest]
Signed-off-by: Stephen Warren <swarren@nvidia.com>