Add plymouth_msg

[uecasm]

Changes proposed in this pull request:

• Adds plymouth_msg to kiwi-repart

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This is mostly a minimal proof of concept; it can be merged as is but I expect you might want to move plymouth_msg to the library and then use it in other places too.

The core idea is that I noticed on large disks the first-boot repartitioning could take long enough that people might wonder what's going on, especially on systems running plymouth. This adds a simple message that displays on plymouth to let people know what it's doing (and implicitly that subsequent boots should be faster).

One caveat is that at least on current versions of OpenSUSE (tested in 15.6 but at time of writing it hasn't been fixed in later versions) there's a bug in plymouth-dracut which means that label-pango.so is not installed to the initrd and thus no text will display. This can either be fixed by patching /usr/lib/plymouth/plymouth-populate-initrd to install label-pango.so instead of label.so, or by adding an equivalent entry to any dracut module, e.g. install_optional_items+=" /usr/lib64/plymouth/label-pango.so ".