Clarification / button needed for ISO boot options

Thanks again for the great software!

I wanted to turn off the Ubuntu splash screen, and show the usual boot messages, when booting the ISO in a virtual machine; and I kept on doing something WRONG: I kept on opening Console, changing `/etc/default/grub`, inserting "noplymouth" in `GRUB_CMDLINE_LINUX_DEFAULT=` and/or `GRUB_CMDLINE_LINUX=`; then running `update-grub`; and finally exiting the Console and clicking on Build. Needless to say, that was completely ignored when booting a resulting ISO in a virtual machine - and the splash screen appeared anyway.

After some digging through the source, it turns out `/usr/share/ubuntu-builder/extras/Build` calls `genisoimage` with parameters ` -b isolinux/isolinux.bin -c isolinux/boot.cat`; looking further based on this, I finally found that I should change the file `ISO/isolinux/txt.cfg` in the `/home/ubuntu-builder` directory, and insert the "noplymouth" there; then if I click on Build, and boot the resulting ISO in a VM - finally I get what I expect: no splash screen, and normal boot messages.

I'd suggest adding an additional button (or a menu option), called something like "ISO/CD boot settings"; upon clicking this, just the chroot terminal can be started, with a starting message like: "To change CD boot options (such as disabling the splash screen), take a look at the file ISO/isolinux/txt.cfg" - I'd need nothing more than this.

Thanks!