Mark DISTRO_DEFAULTS as deprecated

Standard boot has been in place for a while now. Quite a few problems
have been found and fixed. It seems like a good time to mark the
script-based approach as deprecated and encourage people to use standard
boot.

Update the DISTRO_DEFAULTS Kconfig to encourage people to move to
standard boot, which is able to boot Linux distributions automatically.

Add a short migration guide to make this easier.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>