sl: new package

SL (Steam Locomotive) runs across your terminal when you type "sl" as
you meant to type "ls". It's just a joke command, and not useful at
all.

[Thomas:
 - Change "depends on BR2_PACKAGE_NCURSES" to "select
   BR2_PACKAGE_NCURSES".
 - Rewrap Config.in help text.
 - Remove comment in help text about the need to check dependencies,
   there's nothing to be done really.
 - Change the license from "Unclear" to "Custom".
 - Use $(TARGET_CONFIGURE_OPTS instead of passing CC and CFLAGS
   explicitly.
 - Pass '-m 0755' when doing the $(INSTALL)
 - Add ncurses to SL_DEPENDENCIES.]

Signed-off-by: Jens Rosenboom <j.rosenboom@x-ion.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>