boot tagging: minor fixes

msgbufinit may be called multiple times as we initialize the msgbuf into a
progressively larger buffer. This doesn't happen as of now on head, but it
may happen in the future and we generally support this. As such, only print
the boot tag if we've just initialized the buffer for the first time.

The boot tag also now has a newline appended to it for better visibility,
and has been switched to a normal printf, by requesto f bde, after we've
denoted that the msgbuf is mapped.