helper: Make unhexify() robust on invalid data

The current implementation is not suitable for user provided data
because it does not detect invalid inputs in many cases. For example,
the string "aa0xbb" is successfully converted to the 3 bytes: 0xaa,
0x00 and 0xbb. An other example is "aabi" which is successfully
converted to the 2 bytes: 0xaa and 0x0b. Both are obviously incorrect.

Make unhexify() robust on invalid data and use more appropriate data
types for its parameters. Also, add a small documentation for the
function.

Change-Id: Idb799beb86fc608b066c8a76365021ed44c7f890
Signed-off-by: Marc Schink <openocd-dev@marcschink.de>
Reviewed-on: http://openocd.zylin.com/3792


Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>