Add md5 command to toolbox.

This command outputs the MD5 for specified files.  The output is
in the same form as the md5sum command on Linux.

Signed-off-by: Scott Anderson <saa@google.com>

(cherry picked from commit d0455c95)

Change-Id: I3be7751b6e301eb2d12844f88f1f5b9ed1225f6b