Fix handling of zipl.conf in plain zipl bootloader

[schaefi]

When using the plain zipl bootloader kiwi created a /etc/zipl.conf file. However, this file was only useful during image build as it points to a loop target device and geometry but does not represent a proper config file to be used in the running system. In addition the different distributors provides their own version and layout of the zipl.conf to be used inside of the system and with their respective tools. Thus this commit changes the way how kiwi operates in a way that the zipl.conf used in the initial image only exists during the image build process. An eventual present /etc/zipl.conf will not be touched by kiwi. This Fixes #2597

[schaefi]

This PR must be used in Staging to check that no integration test broke. No merge before we have a green check mark here.

[schaefi]

ok integration test looks good

[  167s] [ INFO    ]: 11:10:10 | Creating zipl bootloader configuration
[  172s] [ DEBUG   ]: 11:10:15 | EXEC: [chroot /var/tmp/kiwi_mount_manager.iayfardu zipl --noninteractive --config /kiwi_zipl.conf_uf8ecf9d --blsdir /boot/loader/entries --verbose]

[Conan-Kudo]

shellcheck Expected — Waiting for status to be reported seems to be stuck, so I'm force merging it.