log: fix kept_alive() and report expired timeout

log: fix kept_alive() and report expired timeout

The kept_alive() function is called to inform the keep-alive code
that a keep-alive as been just kicked through some other path.
But kept_alive() erroneously resets the timeout counter without
checking if it has already expired, thus masking a potential
timeout.

Check if timeout counter has expired during kept_alive().
While there, put the timeout values in macros and explicit the
units in the timeout messages.

Change-Id: Iaf6368b44e5b5352b1cc4e7efbb2368575dcfa08
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5705
Reviewed-by: <jonatan_zeidler@gmx.de>
Tested-by: jenkins