package/xbmc: use poweroff to poweroff the RPi

Using "poweroff" instead of "halt" has the side advantage of flashing
the "ACT" LED 10 times (@ ~1Hz) to instruct the user when it is safe
to unplug the power supply.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>