Add information about kernel console tty to each machine size

[simcod]

Hardware vendors might expose the serial port on different ports. In order to use multiple machine sizes from different vendors in a partition, it should be possible to specify the kernel tty individually for each machine size.

Currently, this information is stored in the partition within the bootconfig. It is used by the kernel of the metal-hammer. Because pixiecore does not know anything about machine sizes, the information need to remain there.

However, while installing the target OS the kernel console tty argument can be retrieved from the metal-api for the used machine type. The partition kernel parameter should be used as default unless otherwise specified within the machine size.

[simcod]

We found out, that the console parameter for the kernel can be specified via iPXE. Thus, it is not necessary to add this information to the machine size.
Related pull request in the pixiecore: metal-stack/pixie#43