Add SEAPATH customization file

[dupremathieu]

Modify SEAPATH to support custom variables. Custom variables can be defined in the seapath.conf file, in the environment or in Yocto local.conf file.

All SEAPATH custom variables begin with SEAPTH_.

All available variables are described in the seapath.conf.sample file.

These variables are Yocto variables with Yocto override syntax support.

Refactor wic, machine and images:

• remove no-iommu images. Using is enough to generate an image without IOMMU support enabled.
• do not define kernel parameters in WIC files and remove wic files becoming useless
• remove votp-vm to votp-guest for coherency
• create the votp-flash machine
• remove votp machine and create votp-host machine instead

close #27 close #53