Removed stale/incomplete UART service code from the bootloader kernel.

Changes:
- soundMixerSaw.s no longer polls UART during HSYNC/audio timing.
- Restored the original fixed WAIT delays (39 and 66 cycles) in those slots.
- Removed stale UartInitRxBuffer export.
- PD1 is no longer configured as UART TX output and PD0 no longer gets the UART RX pull-up.
- The full-speed SPI with CRC-triggered half-speed fallback remains unchanged.
