Comment 7 for bug 1541486

This issue also happens when upgrade from 15.10 to 16.04. From apt-term.log, dpkg tries to configure initramfs-tools before plymouth-theme-ubuntu-text, which actually generate /usr/share/plymouth/themes/ubuntu-text/ubuntu-text.plymouth in configure script.

I think there're 2 ways to fix this problem.
1.Make dpkg configure plymouth-theme-ubuntu-text before initramfs-tools
2.Make plymouth's initramfs hook also looks at old file location.
I think 2nd way is more doable, although I don't have experience with dpkg package ordering.