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>