This reverts commit 0dac4d38.

Merge branch 'feature/15985-reproducible-usb-image-intrigeri' into feature/15292-usb-image (Closes: #15985)

Merge remote-tracking branch 'origin/doc/16006-usb-images' into feature/15292-usb-image (refs: #16205)

Merge remote-tracking branch 'origin/feature/15998+verification_ext_design' into feature/15292-usb-image (Closes: #15998)

Thanks segfault for the suggestion.

This reverts commit 535a2d38.

On this devel (= non-frozen) branch, we would like to get rid of the
freeze exception that was granted to linux 4.18.20-2. So besides the
revert-the-revert dance explained in 535a2d38, this commit simply
lifts the freeze exception that was granted for the 3.11 release.

Conflicts:
	debian/changelog

This reverts commit 04970143.

I failed to capture the post-release instructions; let's keep the freeze
exceptions on this stable/frozen branch. Sorry for the confusion and the
revert-the-revert dance…

Conflicts:
	debian/changelog

This reverts commit c55d08e9.

We already pass the --invariant option with sets a fixed volume ID:

	case OPT_INVARIANT:
	    invariant = 1;
	    volume_id = 0x1234abcd;

It's not needed anymore with mtools 4.0.18-2.1.

Let's avoid a potential cause of build reproducibility trouble,
by ensuring no uninitialized memory can sneaking in there.

Let's make it easier to spot issues, such as passing an invalid
value to -i, which could make the USB image unreproducible.

This should fix:

  gpg: cannot open '/dev/tty': No such device or address

Fix copied from f9605653.

This problem will start affecting any branch that uses a recent APT snapshot for
the "debian" archive in the Vagrant build VM.

It makes it hard to generate a reproducible FAT filesystem.
Instead, use Mtools run under faketime, which has no such problem
according to our preliminary tests.

To simplify the migration of the script to this new method, drop obsolete
bootloader configuration mangling code:

 - We don't ship EFI/BOOT/grub.conf. That must be a leftover of Fedora's
   liveusb-creator code.

 - Since 1ac411ac (Tails 3.3), we don't need to
   rename EFI/BOOT/isolinux.cfg to EFI/BOOT/syslinux.cfg anymore.

 - Since 4ffb8676 (Tails 0.14), we don't need to
   adjust absolute paths to the "isolinux" directory in the syslinux
   configuration files anymore.

To simplify the migration of the script to this new method, also:

 - Stop changing file permissions. I could not find what this code is for
   as it was part of the initial version of the script in tails.git and
   I doubt we need it in the context of the Tails ISO + USB image build.

 - Don't reset timestamps: this should not be needed anymore since
   all files on the FAT filesystem are created with Mtools + faketime,
   so their timestamps should be SOURCE_DATE_EPOCH.

Requested-by: goupille (for frontdesk).