stub __libc_start_main

We may have to implement it in the future if we support static PIEs, but for
sure not at the moment, where we handle all the initialization ourselves.
Still, objects without lazy linking will abort if the symbol is not present.

Reviewed-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Glauber Costa <glommer@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
dhcp: Fix MTU logging

When OSv started, using dmesg command we will see a message such as:

  Configuring eth0: ip 192.168.122.15 subnet mask 255.255.255.0 gateway 192.168.122.1 MTU 0

We found that the dhcp packet doesn't carry a positive interface MTU. In
order to avoid misunderstanding, change the MTU value of displaying
message to ETHERMTU(1500) when dhcp packet's MTU is zero.

Signed-off-by: wangbicheng <wangbicheng@huawei.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>