Add SECO i.MX8 machines and update setup-environment script

About update setup-environment script: This addition considers the scenario in which also a custom layer, containing the MACHINE in use, is present. In this case may be happen that the SRCREV.conf file to use is stored inside that folder, so the first place in which the algorith searches the file is the custom layer; if the file is not present, the second place in which to look for is the architecture specific layer. This allows to fix the revision of some repositories directly into the custom layer.