Enable RTT-based terminal

[henrygab]

The benefit is that a developer would only need to connect the SWD port (and power), and have:

1. Debugging
2. Programming new firmware
3. Interactive Terminal

All through the single SWD connection.

This is not technically difficult. The firmware already has some basic concept to redirect the USB CDC to a UART. This would extend this concept.

At present, RTT Channel 0 exists and is setup for bi-directional communication, but no code checks for input from the host.

• Adjust debug_rtt.h so that printf-style macro prints to a per-core buffer, prefixed with a non-zero virtual terminal indicator and ending with a zero virtual terminal indicator. (VT indicator is just byte 0xFF followed by a byte in range 0x00..0x0F to indicate the virtual terminal).
• Hook the printf() style output to also output via RTT
• Hook the getc() style input to also accept input from RTT

As an optimization / stretch goal:

• Format strings once into the per-core buffer, with VTT prefix and postfix
• Send the formatted string via RTT
• Overwrite the postfix with NULL
• Send the formatted string (starting at offset 2) to USB CDC and/or UART (per config)