configs/qemu: enable dhcp on network interfaces

Enable dhcp networking for qemu configs considered good enough for
testing purposes.

This excludes:
arm_nuri - emulation doesn't seem 100% correct for networking.
ppc_virtex_ml507 - doesn't emulate networking.
sh4eb - emulation doesn't seem 100% correct for the NIC.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Reviewed-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>