target/target.c: adding keep_alive() to while loop.

Adding the call to keep_alive() to suppress warnings when
running the async flash algorithm. Issue observed when
loading large pieces of code on slower debuggers.

Change-Id: I7660fa05f68ebd7be07b2ca0a55b0f3b6ae718f3
Signed-off-by: Kevin Gillespie <kgills@gmail.com>
Reviewed-on: http://openocd.zylin.com/4686
Tested-by: jenkins
Reviewed-by: Jesse Marroquin
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>