system/ymodem: fix serial reliability and add optimize rb mode

[yangsong8-a1]

Summary

This PR includes two improvements to the ymodem sbrb.py tool:

1. Fix serial reliability issues: Add flush and reset input buffer operations when sending commands to ensure reliable ymodem transfer initiation over serial connections.
2. Add bootloader burn support: When a device automatically enters ymodem rb mode at startup, the PC tool no longer needs to send the rb command. Instead, it waits for a specified number of consecutive 'C' characters before continuing with the ymodem protocol.

Impact

• Stability: Improves reliability of ymodem transfers by properly clearing serial buffers
• Compatibility: Adds new -c and -w options for bootloader burn scenarios without breaking existing functionality
• Code Quality: Clean implementation following existing code patterns

Testing

Build Host: Ubuntu 22.04 x86_64
Target: tc4dx evb

Test Steps:

Functional test on real hardware (bootloader burn scenario):

• Device enters ymodem rb mode automatically at boot
• PC waits for 3 consecutive 'C' characters with 30s timeout

$ python3 sbrb.py -s /dev -t /dev/ttyUSB0 -b 230400 -k 16 -c 3 -w 30 vela_core0.bin
sending
name:vela_core0.bin filesize:1818624
100.0% 1818624:1818624 left:00h00m00s
time used:85.8s speed 20.7kB/s

 all time:85.82s average speed:20.7kB/s