I found that the line GRUB_CMDLINE_LINUX_DEFAULT= (must have added) " … iommu=soft" at /etc/default/grub
And I have just discovered that after purchasing a Gigabyte 970 gaming mainboard with the AMD 8350FX CPU (cheap and good enough for me).
And not having ethernet connection with Manjaro and with the liveISO.
I suppose if the LiveISO adds this iommu=soft the ethernet would work, or if this iommu=soft, is not all right for all the boards with iommu, some kind of detection in order to change this parameter to those (I suppose this is not the only one) that need this change or option.
If no change is made, at least I hope that this post will help others. and remember to update grub after editing that line.