keep gdb aware of threads if RTOS is set but no threads are created

if gdb server (openocd) answers qfThreadInfo with an empty string during boot,
gdb will not request thread info anymore. to keep thread awareness in gdb,
we have to answer with a non empty string, 'l' indicates an end of list,
and is a valid answer here.

Change-Id: I7870a5db1090c786f306db16a25871e69b8a9760
Signed-off-by: Zied Guermazi <guermazi.zied@gmail.com>
Reviewed-on: http://openocd.zylin.com/1432
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>