Allow specifying multiple output consoles for grub

[alexiri]

We'd like to be able to specify multiple output consoles for grub in our Kiwi builds, specifically console and serial.

Grub itself supports specifying multiple output terminals separated by spaces:

‘GRUB_TERMINAL_OUTPUT’

    Select the terminal output device. You may select multiple devices here, separated by spaces. 

(from https://www.gnu.org/software/grub/manual/grub/html_node/Simple-configuration.html)

In Kiwi, space separation has been "abused" to differentiate output and input consoles, so I guess the next best thing would be comma separated values:

<bootloader name="grub2" bls="true" console="console,serial console" timeout="5"/>

This could get kind of confusing, so it might be better to separate the consoles like this, but that would be a bigger change:

<bootloader name="grub2" bls="true" output_console="console serial" input_console="console" timeout="5"/>

[schaefi]

Yeah this topic has been addressed in #2420, there is also a rough explanation why we stick with just one attribute so far. I agree with you that the cleaner solution would be multiple attributes. But so far it had little priority