jtag: commands: save a pointer to list tail

jtag: commands: save a pointer to list tail

By saving a pointer to the tail of the list,
we don't need to traverse the entire command
queue before we're able to append an item to
it.

With this patch, I see a 10% improvement when
using the embedded XDS100v2 on AM437x IDK board
to load a 4MiB binary (linux zImage) to DDR
with load_image.

IOW, we went from ~305kB/sec to ~336kb/sec.

Change-Id: Idb55d49f0d0106043374ab520b2f3b6b32f2c50f
Signed-off-by: Felipe Balbi <balbi@ti.com>
Reviewed-on: http://openocd.zylin.com/2709
Tested-by: jenkins
Reviewed-by: Stian Skjelstad <stian@nixia.no>
Reviewed-by: Daniele Emancipato <daniele12457@hotmail.com>
Reviewed-by: Matthias Welwarsky <matthias@welwarsky.de>
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>