rockchip: binman: Correct the OS prop for U-Boot

rockchip: binman: Correct the OS prop for U-Boot

The U-Boot image is currently being identified as an invalid OS in
spl_fit_image_get_os() due to case sensitive compare.

Use the correct lower-case value to fix this.

Fixes: e0c0efff2a02 ("rockchip: Support building the all output files in binman")
Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
Reviewed-by: Quentin Schulz <quentin.schulz@cherry.de>