rockchip: firefly: Set the environment variable 'usbkbd' to the stdin

rockchip: firefly: Set the environment variable 'usbkbd' to the stdin

Add the 'usbkbd' environment variable to the 'stdin', the contents of
the keyboard input can be auto-displayed on the serial terminal,so
you don't need to manually set the environment variable 'stdin'.

Signed-off-by: Leo Wen <leo.wen@rock-chips.com>
Acked-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>