keep-alive: drop link with log framework

keep-alive: drop link with log framework

OpenOCD implements the GDB keep-alive by sending empty strings as
output for GDB client. This has been implemented as part of the
log framework, creating an odd dependency.

Move the keep-alive notifications out of log framework.
For the moment, keep keep_alive() inside log.c, but it should be
moved in server.c

This should also fix an old issue with KDE Konsole when tab alert
for activity is enabled. The empty strings is sent to all the
connections, including telnet, and causes the tab running OpenOCD
telnet to continuously show activity even when no new text is
printed. Anyway, I cannot replicate this issue anymore.

Change-Id: Iebb00b00fb74b3c9665d9e1ddd3c055275bfbd43
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/6840
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>
Tested-by: jenkins