gdbserver: use common hexify/unhexify routines

Change-Id: I9989b625666e9c60ec9867cf6f4d94f41c998c3f
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/1105
Tested-by: jenkins
Reviewed-by: Mathias Küster <kesmtp@freenet.de>
Reviewed-by: Øyvind Harboe <oyvindharboe@gmail.com>