helper: Code cleanup for hexify()

helper: Code cleanup for hexify()

Simplify hexify() and do not longer use 0 as special case for the
parameter 'count' to determine the string length of the binary input.
Instead, use strlen() outside of the function if needed.
Additionally, fix the return value and return the length of the
converted string. The old function always returned 2 * count.

Also, use more appropriate data types for the function parameters and
add a small documentation.

Change-Id: I133a8ab786b8f7c1296afcaf9c0a0b43881e5112
Signed-off-by: Marc Schink <openocd-dev@marcschink.de>
Reviewed-on: http://openocd.zylin.com/3793
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>