listener print strings

[dagar]

Example

nsh> listener log_message

TOPIC: log_message instance 0 #1
log_message_s
        timestamp: 4128786 (55.251776 seconds ago)
        severity: 3
        text: "[pmw3901] driver start failed"

nsh> listener task_stack_info

TOPIC: task_stack_info instance 0 #1
task_stack_info_s
        timestamp: 128244081 (0.845543 seconds ago)
        stack_free: 428
        task_name: "px4io"

[dagar]

@julianoes can you try this again with your particular toolchain?

[julianoes]

Looks fine.

[julianoes]

Builds fine with GCC 9.1.1 and clang 8.0.0.

I'll check at home on Arch as well later but I doubt it's newer.