specialize overlay script & refactor

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

initramfs/ubuntu/hooks/grub-btrfs-overlay

@@ -12,3 +12,4 @@ esac
 
 . /usr/share/initramfs-tools/hook-functions
 manual_add_modules overlay
+copy_exec /usr/bin/findmnt /usr/bin

initramfs/ubuntu/scripts/local-bottom/grub-btrfs-overlay

@@ -1,30 +1,32 @@
 #!/bin/sh -e
 PREREQ=
 prereqs() {
-  echo "$PREREQ"
+	echo "$PREREQ"
 }
 case $1 in
-  prereqs)
-    prereqs
-    exit 0
-    ;;
+	prereqs)
+		prereqs
+		exit 0
+		;;
 esac
 
 . /scripts/functions
 on_err() {
-	panic 'script error'
+	log_failure_msg 'error setting up overlay'
 }
 trap on_err ERR
-if [ "$readonly" = y ]
+if [ -x /usr/bin/btrfs -a -x /usr/bin/findmnt ] &&
+		 [ "$(findmnt -no FSTYPE -M "$rootmnt")" = btrfs ] &&
+		 [ "$(btrfs property get $rootmnt ro)" != ro=false ]
 then
-	log_begin_msg 'remount read-only root as read-only layer in non-persistent, writable overlay'
-	lower_dir="$(mktemp -d -p /)"
-	ram_dir="$(mktemp -d -p /)"
+	log_begin_msg 'remount read-only subvolume as read-only layer in non-persistent, writable overlay'
+	trap log_end_msg EXIT
+	lower_dir="$(mktemp -dp /)"
+	ram_dir="$(mktemp -dp /)"
 	upper_dir="$ram_dir"/upper
 	work_dir="$ram_dir"/work
 	mount --move "$rootmnt" "$lower_dir"
 	mount -t tmpfs cowspace "$ram_dir"
 	mkdir -p "$upper_dir" "$work_dir"
 	mount -t overlay -o lowerdir="$lower_dir",upperdir="$upper_dir",workdir="$work_dir" rootfs "$rootmnt"
-	log_end_msg
 fi