target: clear running_alg flag after reset

target: clear running_alg flag after reset

After the target was reset we can be sure it's not running any
algorithm.

This fixes the following failure scenario:

On my STM32F103 board after I start the firmware and then stop and try
to "load" in gdb (before doing mon reset halt), I get

Error: timeout waiting for algorithm, a target reset is recommended

However, target reset doesn't help as the flag is still there ("Error:
Target is already running an algorithm"), so I have no choice but to
restart the OpenOCD process.

I'm not sure yet what exactly prevents load from working after my
firmware is initialised, most probably some interrupt is firing and my
handler produces a fault due to garbled RAM.

Change-Id: Idd977f2780a64d84800e3abd412cffc1ab6801b0
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/1512
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>