busybox: adjust configuration to add fancy options to the sleep applet

Set CONFIGURE_FEATURE_FANCY_SLEEP and CONFIG_FEATURE_FLOAT_SLEEP so that sleep accept floating point arguments.
This add 492 bytes to the busybox binary compiled for arm or 2748 bytes when compiled for i386.

Signed-off-by: Sylvain Raybaud <sylvain.raybaud@green-communications.fr>
Reviewed-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>