use u-boot-rockchip.bin to copy SPL/TPL/U-Boot to the image.
since binman was used in mainline u-boot for rockchip, we can use
u-boot-rockchip.bin instead of idbloader.img and u-boot.itb.
Reviewed-by: Tianling Shen <cnsztl@immortalwrt.org>
Signed-off-by: FUKAUMI Naoki <naoki@radxa.com>
Link: https://github.com/openwrt/openwrt/pull/15815
Signed-off-by: Robert Marko <robimarko@gmail.com>