rpi3: minor: add output to both monitor and UART

Add support of kernel output both to connected monitor and UART
Add "usb start" at pre-boot stage to be able to use a keyboard in U-Boot

Fixes: https://github.com/OP-TEE/optee_os/issues/1367
https://github.com/OP-TEE/build/issues/131



Acked-by: Jens Wiklander <jens.wiklander@linaro.org>
Reviewed-by: Joakim Bech <joakim.bech@linaro.org>
Signed-off-by: Igor Opaniuk <igor.opaniuk@linaro.org>