[Feat] Increase transfer speed

[vicgwee]

Currently, the transfer speed is about 25-27 KB/s.

Already tried increasing baudrate in #49, max was 400k.

[vicgwee]

Did some testing, speed increases linearly with baudrate. So the baudrate is the limiting factor.

Tried the following tweaks:

• increase SERIAL_BUFFER_MAX_SIZE -- doesn't affect
• increase TXBSIZE and size of oneKbuf to 2048 -- doesn't affect
• set Txwindow to values greater than 0 -- reduced transfer speed
• increase baudrate to 480k, 640k, 1M ... -- cli doesn't reliably work

According to this, the max baudrate is 500k. However I tried and the CLI didn't reliably work either. Sticking to 400k might be safer, since it's a factor of the Apollo 3's clockrate (96 MHz).