[RPI][UART][UBOOT] direct U-boot console to UART instead of suppressing it entirely
The U-boot output was suppressed to prevent it from covering the boot logo. See: !2 (d7cbf00f) These commits:
- re-enable the u-boot console output;
- enable the UART (GPIOs 14 and 15 on rpi4) by default;
- direct the console output to the UART only.