Use $() style command substitutions.
Do not require the Bash shell.
Use && || list control over -a -o test operators.
Remove unused exists() function.
Collect kernel command line once into a variable, and
  perform more robust parsing.
Take advantage of losetup -f --show for assignment
  of variables for loop devices.
Use ${mountopts:+$mountopts} expansion to nothing
  if mountopts is unset or empty in livesys-main.
Use ${USERADDARGS:+$USERADDARGS} expansion to nothing
  if USERADDARGS is unset in livesys-main.
Drop the unneeded -n (not zero-length) operator in test
  expressions, and use ! instead of -z (zero-length).
  This results in quicker-to-understand code as it
  more closely represents the variable object instead of
  the testing methodology and its double negatives.
Clearly demark the run-on-first-boot-only code from
  the rest in livesys-main.
Move, and comment to keep, the /.liveimg-configured flag
  file at the end of the livesys-main script, so that it
  is not set prematurely in a faulty script invocation.
Replace touch with : > file syntax.
Include a switch for rd.live.debug in livesys-main & livesys-late.
Provide a filesystem check/repair function in functions
  and use it on a persistent home.img or home device filesystem.
Also In functions:
Update the strstr function to a simpler version from dracut.
Drop the dependency on grep by using shell substring removal.
Drop the Bash syntax $"..." for string translation for
  portability & security reasons. See
  https://www.gnu.org/software/gettext/manual/html_node/bash.html