gdb_server: simplify logic to enable/disable gdb_log_callback()

gdb_server: simplify logic to enable/disable gdb_log_callback()

GDB client cannot always display generic messages from OpenOCD.
The callback gdb_log_callback() is continuously added and removed
to follow the GDB status and thus enabling/disabling sending the
OpenOCD output to GDB.
While this is a nice stress test for log_{add,remove}_callback(),
it is also a waste of computational resources that could impact
the speed of OpenOCD during GDB user interactions.

Add a connection-level flag to enable/disable the log callback and
simply change the flag instead of adding/removing the callback.

Use an enum for the flag instead of a bool. This improves code
readability and allows setting other states, e.g. keep-alive
through asynchronous notification https://review.openocd.org/4828/

Change-Id: I072d3c6928dedfd0cef0abe7acf9bdd4b89dbf5b
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/6839
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>
Tested-by: jenkins