busybox skeleton: allow logins from ttymxc[0-2]

Add ttymxc[0-2] to the list is the /etc/securetty of the Busybox skeleton.
This is useful for serial logins on i.MX based systems. The same serial
devices already appear in the generic "target_skeleton/".

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>