busybox: improve support for telnetd

If target has connection to the network it might be pretty useful to
have telnet connection to it instead of serial console or even in
addition to serial console.

This changes makes the busybox package automatically install an init
script, and tune the securetty file to make telnetd work when
CONFIG_FEATURE_TELNETD_STANDALONE is enabled in the Busybox
configuration.

[Thomas:
  - don't create a new Buildroot Config.in option, just test if
    CONFIG_FEATURE_TELNETD_STANDALONE is enabled or not in the Busybox
    configuration
  - move the securetty tuning in busybox.mk instead of system.mk
  - use start-stop-daemon in the init script, in order to properly
    implement the stop and restart actions
  - misc other minor improvements in the init script.]
[yann.morin.1998@free.fr:
  - don't use securetty
  - drop stray variable BUSYBOX_SET_STANDALONE_TELNETD]

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Peter Korsgaard <peter@korsgaard.com>
Cc: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>