efi_loader: skip warnings for network configuration

Skip messages should only be written if the setup is not suitable for
testing.

If DHCP is enabled, we should not write a skip message if no static network
configuration is supplied.

Likewise if a static network configuration is supplied, we should not write
a skip message if DHCP is not enabled.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>