target: Fix read_sysctl()

Add a leading "/" so the path is absolute.

Author: douglas-raillard-arm

devlib/target.py

@@ -1183,7 +1183,7 @@ async def read_sysctl(self, parameter):
         """
         Returns the value of the given sysctl parameter as a string.
         """
-        path = self.path.join('proc', 'sys', *parameter.split('.'))
+        path = self.path.join('/', 'proc', 'sys', *parameter.split('.'))
         try:
             return await self.read_value.asyn(path)
         except FileNotFoundError as e: