optimize: replace while loop by memcpy

optimize: replace while loop by memcpy

There is no need to use a while loop here. This patch simple copy
the last bytes with the system function.

Change-Id: Ibda72dca449746efeba5a1af2e45c5990f9cf347
Signed-off-by: Mathias K <kesmtp@freenet.de>
Reviewed-on: http://openocd.zylin.com/364
Tested-by: jenkins
Reviewed-by: Øyvind Harboe <oyvindharboe@gmail.com>
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>