rockchip: Make it possible to define per board boot_targets

Some board may want to have a different boot priority(a laptop
may want to usb has the highest boot priority for third-part
os installation). So let the board can define it's own
boot_targets.

Signed-off-by: Andy Yan <andyshrk@163.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
1 file changed