install.go support more console definitions

[majst01]

if more than one console was configured as metal-kernel cmdline, install-go fails with:

Serial console could no be split into host and port 

[Gerrit91]

Have some questions.

• How to separate multiple consoles in the console string? Or do we need to migrate to a string slice (breaking)?
• Do we want to require a serial console (ttyS) in the kernel string for grub or can it also go without? The check that the given console is of type serial ttyS should already be done in the current implementation, I guess?

[majst01]

Have some questions.

• How to separate multiple consoles in the console string? Or do we need to migrate to a string slice (breaking)?
• Do we want to require a serial console (ttyS) in the kernel string for grub or can it also go without? The check that the given console is of type serial ttyS should already be done in the current implementation, I guess?

I think the simplest approach would be to configure grub to only spit out to ttyS, but what to do if someone defines more than one ttyS

[Gerrit91]

According to the kernel docs, defining multiple serial consoles is not valid:

Note that you can only define one console per device type (serial, video).

(https://docs.kernel.org/admin-guide/serial-console.html)

However, if we allow multiple console, we will also have a broken kernel commandline:

fmt.Sprintf("console=%s", i.config.Console), <-- this has to become a repeated arg