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 exit
in case 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.