rockchip: Use BOOTSTD_DEFAULTS if not DISTRO_DEFAULTS

When we do not enable DISTRO_DEFAULTS (generally, to get distro_bootcmd)
we instea do want to imply BOOTSTD_DEFAULTS so that when using bootstd
the general distro boot functionality will still work.

Signed-off-by: Tom Rini <trini@konsulko.com>