Clean up const usage to avoid excessive casting

Clean up const usage to avoid excessive casting

Don't use const on pointers that hold heap allocated data, because that
means functions that free them must cast away the const.

Do use const on pointer parameters or fields that needn't be modified.

Remove pointer casts that are no longer needed after fixing the constness.

Change-Id: I5d206f5019982fd1950bc6d6d07b6062dc24e886
Signed-off-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Reviewed-on: http://openocd.zylin.com/1668
Tested-by: jenkins
Reviewed-by: Mathias Küster <kesmtp@freenet.de>
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>