Provide od+sed replacement for the bin2char helper

Provide od+sed replacement for the bin2char helper

Using custom build-time tools is always more problematic, especially
for cross-compiling.

This alternative implementation assumes "od" (IEEE Std 1003.1-2001)
and sed are available which should be the case for any reasonably
modern system.

Change-Id: I0208f475648c78e7dca127ff4bab60d314b2bf53
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/2139
Tested-by: jenkins
Reviewed-by: Fatih Aşıcı <fatih.asici@gmail.com>
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>