rockchip: fix boot from non-MMC devices

rockchip: fix boot from non-MMC devices

Booting from non-MMC devices on Rockchip targets without this
change results in a boot failure:

Model: FriendlyElec NanoPi R5S
Net:   eth0: ethernet@fe2a0000
Hit any key to stop autoboot:  0
** Booting bootflow 'nvme#0.blk#1.bootdev.part_1' with script
** No partition table - mmc 0 **
** No partition table - mmc 0 **
Couldn't find partition mmc 0:1
Can't set block device
Wrong Image Type for bootm command
ERROR -91: Protocol wrong type for socket: can't get kernel image!
Boot failed (err=1)

This change fixes the default boot script for Rockchip targets to
support booting from non-MMC devices such as NVMe or USB drives.

Some targets with only a boot rom (e.g. NanoPi R5S) may require u-boot
to be installed on the eMMC or a MicroSD card in order to boot from
non-MMC devices.

Fixes: #14420
Reviewed-by: Tianling Shen <cnsztl@immortalwrt.org>
Signed-off-by: Justin Klaassen <justin@tidylabs.app>