efi_loader: efi_net: Add support for multiple efi_net_obj

efi_loader: efi_net: Add support for multiple efi_net_obj

Add support for multiple efi_net_obj structs in efi_net.c. This comes
in preparation for an EFI network driver supporting multiple network
interfaces. For now the EFI network stack still registers a single ethernet
udevice as an EFI network device even if multiple are present, namely
the one that was the current device at the moment of EFI initialization.

Signed-off-by: Adriano Cordova <adriano.cordova@canonical.com>