systemd: update_state_probe: Use a file mutex to avoid race condition

As soon as the UUID is modified udev re-runs the rules for the partition.
However, the rule expects the new root UUID to be cached in a file, and
if the udev rule gets there before the file is created it fails.

This commit waits on a lock file mutex before accessing said file.

Change-type: patch
Signed-off-by: Alex Gonzalez <alexg@balena.io>

meta-balena-common/recipes-core/systemd/systemd/resin_update_state_probe

@@ -39,6 +39,8 @@ valid_device() {
 # If the UUIDs have been regenerated in this boot, the root UUID is stored
 # on a temporary file, otherwise fetch from the kernel command line
 if [ -d "/run/initramfs" ]; then
+	# Avoid a race condition between udev and writing the file
+	wait4rm "/run/initramfs/bootparam_root.mux" 30
 	new_root="/run/initramfs/bootparam_root"
 else
 	# Older releases do not create /run and use /tmp instead