Minor tweaks to terminal output, variables, and GRUB stanza

[iskunk]

This is just a few miscellaneous updates to 42_grml to smooth the way to the next PR, which is the big one.

• Print Found grml-rescueboot ISO image: instead of Found Grml ISO image: as the latter implies that the ISO image comes from Grml, whereas the former is more to the point of "grml-rescueboot found this image";
• Similarly, just print the ISO filename as the title, as "Grml Rescue System" implies that the ISO is the actual Grml rescue system;
• Use the existing ${indent} variable instead of typing out more spaces;
• Store "${rel_dirname%/}/${grml}" in a variable, as it will be needed more than once;
• Add Debian bug reference for the tpm issue, as it is cryptic otherwise;
• Use the existing GRUB variable to set the loopback path;
• Perform cleanup after exiting the ISO loopback menu.

[mika]

Wonderful one, would totally take this as well, though could you please update your commit message and include the information from your PR there (c/p perfectly fine), because that's certainly useful information! Thanks! :)

[iskunk]

No problem! I've added the PR bullet points to the commit message.

[mika]

@iskunk wonderfull, thank you so much! :+1: