add "mrb" command to read a byte of memory into a variable

Change-Id: I5ddc9fbcc55958a249548627bd15824df6dc0d61
Signed-off-by: Matthias Welwarsky <matthias.welwarsky@sysgo.com>
Reviewed-on: http://openocd.zylin.com/3542


Tested-by: jenkins
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>