os-helpers: extend filesystem helper with wait4rm

This function waits until a file is removed or times out - useful to
implement basic file based mutexes.

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

meta-balena-common/recipes-support/os-helpers/os-helpers/os-helpers-fs

@@ -36,6 +36,26 @@ wait4file() {
 	return 0
 }
 
+# Wait for a file to be removed with loop count limit.
+# Use-case example: wait for udev to create a filesystem symlink.
+# Arguments:
+#   1 - target path
+#   2 - number of loops (each loop sleeps for 100ms)
+wait4rm() {
+	_lpath="$1"
+	_lloops="$2"
+
+	while [ -e "$_lpath" ]; do
+		if [ "$_lloops" -gt 0 ]; then
+			sleep 0.1
+			_lloops=$((_lloops-1))
+		else
+			return 1
+		fi
+	done
+	return 0
+}
+
 # Output the UUID for the specified block device.
 # Arguments:
 #   1 - Target block device