command: Convert format-string in rtt

[koendv]

Detailed description

Detailed description

This PR brings the format strings in cmd_rtt up to date, removing gcc -Wformat errors, but keeping existing functionality.
See #1676

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• I've read the Code of Conduct
• I've read the guidelines for contributing to this repository
• It builds for hardware native (make PROBE_HOST=native)
• It builds as BMDA (make PROBE_HOST=hosted)
• I've tested it to the best of my ability
• My commit messages provide a useful short description of what the commits do

Closing issues

[dragonmux]

LGTM, merging. Thanks for the contribution!

[ALTracer]

Yes, thank you, I'm acknowledging this fixes rtt-related format warnings from gdb_outf() in src/command.c when that's declared as void gdb_outf(const char *fmt, ...) __attribute__((format(printf, 1, 2))) (change pending, not in main).