udhcpc doesn't send a hostname by default. Use the system hostname if
nothing else is specified, to always send a hostname.
It syncs the behaviour to odhcpc, which always sends a hostname.
Signed-off-by: Mathias Kresin <dev@kresin.me>
Acked-by: Stijn Tintel <stijn@linux-ipv6.be>
Acked-by: Hans Dedecker <dedeckeh@gmail.com>