This would avoid the undesirable intertactions with the serial driver
described in https://github.com/apache/nuttx/issues/14662.
Although I'm not entirely happy with this fix because it assumes
the particular implementations of up_putc/up_nputc and its association
to the serial devices, I haven't come up with better ideas for now.
An alternative is to place some serializations inside the target
specific serial (and/or whatever provides up_putc api) implementaitons.
But it isn't too attractive to put potentially complex logic into the
low-level machinaries, especially when we have a lot of similar copies
of it.
Another alternative is to deprecate up_putc. (at least for the purpose
of syslog.) But it seems at least some of users are relying on what
the current implementation provides heavily.
This commit also removes g_lowputs_lock because the critical section
would serve the purpose of the lock as well.
f2aeb5e56f