WIP: Update per shellcheck, etc.
Placing PLYMOUTH_POPULATE_INITRD command into existing 'if' statement; adding
exit in case
cd fails; adding '.' in case
find has no default dir; adding quotes to prevent globbing & word splitting. Replacing '[' with '[['; cleaning up quotes; adding
mktemp fails; adding ':?' in case of race condition; '&& ... ||' is not well defined, restructuring into 'if' statement; connecting arguments of
rm -f to dir with './' to prevent globbing.