Restore Tails boot splash on Buster
feature/buster now displays the Buster plymouth theme instead of our ninja theme.
From the .buildlog:
W: plymouth: The plugin label.so is missing, the selected theme might not work as expected. W: plymouth: You might want to install the plymouth-themes package to fix this.
Restore Plymouth theme to "text" (refs: #16743)
The default theme in Buster ("futureprototype") is Debian-branded
and thus unsuitable for Tails. Let's revert to the one we use
in Tails 3.x.