main/u-boot: move rockchip to single image and fix pinebook pro defaults

From the u-boot rockchip documentation [0]:

Package the image with U-Boot TPL/SPL

SD Card

All Rockchip platforms (except rk3128 which doesn’t use SPL) are now supporting a single boot image using binman.

[0] https://u-boot.readthedocs.io/en/latest/board/rockchip/rockchip.html

Tested on a pinebook-pro, with the update-u-boot script defaults fixed.