altos: Keep telemetry and command output from interleaving

altos: Keep telemetry and command output from interleaving

When monitoring telemetry, the frame needs to be sent together on one
line, not split with the output of a command (like 'a') mixed in.

Use a mutex for products with monitoring to keep command output and
telemetry output each on separate lines.

Signed-off-by: Keith Packard <keithp@keithp.com>