helper: hexify correctly handle signed chars

The current implementation of hexify was not correctly handling signed chars.

This function is currently used by the ti-icdi driver and as such was causing
random write issues.

As a note perhaps a better long term fix would be to change to using uint8_t
buffers rather than char. This will require changes to the ti-icdi driver
aswell.

Change-Id: I572e69ff2b99227a7d412de056458c0393794b03
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/1124
Tested-by: jenkins
Reviewed-by: Øyvind Harboe <oyvindharboe@gmail.com>

src/helper/binarybuffer.c

@@ -393,7 +393,7 @@ int hexify(char *hex, const char *bin, int count, int out_maxlen)
 		count = strlen(bin);
 
 	for (i = 0; i < count; i++)
-		cmd_len += snprintf(hex + cmd_len, out_maxlen - cmd_len, "%02x", bin[i]);
+		cmd_len += snprintf(hex + cmd_len, out_maxlen - cmd_len, "%02x", bin[i] & 0xff);
 
 	return cmd_len;
 }