skeleton: add default login port to /etc/securetty

We ran into a "Login incorrect" problem when running the same rootfs
image across platforms with different loging ports ttyS0/1/2/3.

Simply assignning "console" to BR2_TARGET_GENERIC_GETTY_PORT, which in
turn modifies the /etc/inittab, is not enough because the "console" device
was missing in the /etc/securetty.

While current securetty has enumerated a lot of ttys, this patch should save
some efforts to enumerate more.

[Peter: guard with single quotes]
Signed-off-by: Tzu-Jung Lee <tjlee@ambarella.com>
Signed-off-by: Spenser Gilliland <spenser@gillilanding.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>