Kinetis: Flash command function matches datasheet

The kinetis datasheets specify the flash registers as bytes rather
than as words, as the previous implementation did.  This also makes
a few code sections slightly less endian-magical.

Change-Id: If8f4adfc7f4341085ae5b6eacbf7d74bbd74cf08
Signed-off-by: Alex Austin <alex.austin@spectrumdsi.com>
Reviewed-on: http://openocd.zylin.com/1192
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>