busybox: Include 'mesg' utility

The 'dot.profile' supplied by base-files calls 'mesg'.  For simple
images using busybox e.g. core-image-minimal, this utility should be
available to avoid errors on login:

 -sh: mesg: command not found

Provide mesg with busybox.

Signed-off-by: Ash Charles <ashcharles@gmail.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>