shutdown-desktop: use poweroff for the Exec field in the shutdown.desktop

shutdown-desktop: use poweroff for the Exec field in the shutdown.desktop

Use 'poweroff' instead of 'halt' for the Exec field in the shutdown.desktop.

The purpose of this patch is to make the 'shutdown' icon on on our sato
images work as expected for both sysvinit and systemd images. Previously,
the Exec field was 'halt'. The `halt' command could poweroff the system
in sysvinit images but it only could halt the system in systemd images.
The difference is due to the different implementations of the `halt' command.

In sysvinit, the `halt' command will effective execute `/sbin/init 0'. This
is for the compatibility with sysvinit 2.4, as stated in the comments of the
source code. In systemd, the `halt' command will effectively execute the
reboot(RB_HALT_SYSTEM).

As the 'shutdown' icon is expected to actually shutdown the system, we should
use poweroff instead of halt for its Exec field.

[YOCTO #4347]

(From OE-Core rev: c87d44a079c13a98734f9f726ec1b55d86793f8c)

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>