rockchip: rk3399: Drop altbootcmd

The defined altbootcmd was specific to distro_bootcmd which is not
longer in use on these platforms, so drop it.

Tested-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Jonas Karlman <jonas@kwiboo.se>
Signed-off-by: Tom Rini <trini@konsulko.com>