loader: Extend resolution of arg environment variables

Allow resolution of arg enviornment variables when the arg is composed
of values and enviornment varialbles e.g.
(-Djava.rmi.server.hostname=$OSV_IP)

Signed-off-by: Shlomi Livne <shlomi@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
bsd/porting: Adding if_ip function to return an interface ip address

Reviewed-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Shlomi Livne <shlomi@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
loader: Adding OSV_IP environment variable

OSV_IP is set incase there is a single ip address (not including local
interface). When there are multiple ip addresses the environment
variable will not be set.

Signed-off-by: Shlomi Livne <shlomi@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>