Undo custom variable restriction as https://mnt.re/system-image does not anymore link to the latest job in 'main'

Problems with genext2fs:

 - slow O(n^2) runtime
 - need to upgrade ext2 filesystems to ext4 on first boot
 - inode size of 128 disallows timestamps beyond 2038 or sub-second
   precision timestamps

Problems with mkfs.ext4:

 - not bit-by-bit reproducible across multiple invocations

The problems with mkfs.ext4 are solved once this PR is merged:

https://github.com/tytso/e2fsprogs/pull/118

The job artifacts pointed to by

https://source.mnt.re/reform/reform-system-image/-/jobs/artifacts/main/browse?job=build

will come from the latest CI build of the branch with the same top
commit as 'main'. This is a problem as that link is used by
https://mnt.re/system-image.  So even if a pipeline run with custom
variable settings is done on a branch other than 'main', its results
will be put behind above link, thus tainting it.

To prevent this, forbid any pipeline runs with custom variable values in
the 'reform' namespace. To do custom runs, fork reform-system-image
under your own username instead.

mkimage.sh: also build pocket-reform-system-a311d.img, pocket-reform-system-imx8mp.img, reform-system-imx8mp.img

bsdtar can do the same job as unzip but without all the problems that
the unzip CLI comes with. Instead, bsdtar provides an interface which
should be familiar to users of GNU tar and has support for long options.
Problems with the unzip CLI include:

 - the -v (verbose) option prohibits extraction and just lists the zip
   file members (analogous to the -t option of tar)
 - but if -v is given without other arguments prints version
   information
 - by default, unzip applies a filter mangling extracted filenames
   unless the -^ option is used to override this behaviour
 - unzip treats all arguments (zip filenames and member names) as
   wildcards. The man page calls them regular expressions but they are
   just globs.
 - unable to read archives from standard input

For more fun, try to read the unzip man page. Read the man page
especially if Acord, AtheOS, Tandem, VMS, Amiga, OS/2, Windows NT or
MS-DOS are things you have fond thoughts for.

easy custom *.deb repo and creating images for experimental and testing via gitlab pipeline web interface

See merge request !88

allow using packages from a custom reform-debian-packages branch by setting REFORM_DEBIAN_PACKAGES_BRANCH in the gitlab web interface

allow building images for multiple distros

See merge request !87

Extend README

See merge request !86

a311d: fix usb hub reset; ship gpiod

See merge request !84

Build LS1028A System Image

See merge request !83