[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: https://git.seco.com/edgehog/layers/seco/meta-seco-rpi/-/merge_requests/2/diffs?commit_id=d7cbf00f31c37f6a49c39ffd1020bb1ecff8a755 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.