bootctl: fix display of options with embedeed newlines

I have an .efi image with embedded newlinews. Now I don't even remember if it
was created for testing or by accident, but it doesn't really matter. We should
display such files correctly.

(This isn't a problem with normal BLS entries, because input is split into lines
so newlines are consumed.)

(cherry picked from commit 9dae4c8a)