Make the serial console configurable in the `ipxe` template

[damyan]

Currently the serial console is hard-coded in https://github.com/ironcore-dev/boot-operator/blob/main/templates/ipxe-script.tpl#L9.
Console devices can be accessible under a different device names, e.g. /dev/ttyAMA0 for ARM64.

Thus, making the device configurable in the template would be a good enhancement

[damyan]

/cc @5kt @hardikdr