rockchip: rk3399: change boot_target based on u-boot, spl-boot-device

rockchip: rk3399: change boot_target based on u-boot, spl-boot-device

The order distroboot searches for a boot.scr is fixed at compile time.
To make BIOS_DISABLE work as expected and boot from mmc1 instead of
mmc0 if enabled, we need to change the environment at runtime.
Especially as commit: 7ffdda212d ("rockchip: rk3399-puma: add boot-on
regulator to override BIOS_DISABLE") enables the eMMC in U-Boot even
if BIOS_DISABLE is active.

Signed-off-by: Klaus Goger <klaus.goger@theobroma-systems.com>
Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>