kernel parameters normally identify root filesystem as readonly before boot process remounts them writable, so the readonly parameter does not identify readonly filesystems
the script reduces scope to btrfs for simplicity
as a standard linux util, findmnt is copied & used
arrange commands so executables are checked before file system queries and short-circuit evaluation skips unavailable commands
for robustness, trap on error & exit events
replace panic with failure logging, since panic doesn't appear to break to shell
provide a hook to require overlayfs module so the file system is available
define a general script to overlay any readonly root filesystem into a non-persistent, writable filesystem
