rockchip: mkimage: Allow encoding of loader code in spl images

rockchip: mkimage: Allow encoding of loader code in spl images

Rockchip SoCs allow the spl code to be rc4-encoded, not only the
image header, but only newer SoCs allow this encoding to be disabled.

The rk3188 is not part of those and requires its boot code to be
rc4-encoded with the regular key. So add the ability to do this
encoding via a setting on a per-soc basis when building spl images.

Signed-off-by: Heiko Stuebner <[email protected]>
Reviewed-by: Simon Glass <[email protected]>
Reviewed-by: Kever Yang <[email protected]>
Tested-by: Kever Yang <[email protected]>