weston-init: Set $HOME variable for SysVinit manager

SysVinit sets by default the $HOME directory to "/" for the init scripts.
It's picked then by the weston-launcher and make weston-terminal to have
wrongly set $HOME environment variable for the root user.

Signed-off-by: Adrian Fiergolski <adrian.fiergolski@fastree3d.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>