[baseimage]: Fix process-reboot-cause possibly throwing OSError (#3159)

In case of going from previous iteration of SONiC, and the last reboot
was hardware, REBOOT_CAUSE_FILE may not be present and the service may
throw an error.

files/image_config/process-reboot-cause/process-reboot-cause

@@ -109,6 +109,7 @@ def main():
     log_info("Previous reboot cause: {}".format(previous_reboot_cause))
 
     # Remove the old REBOOT_CAUSE_FILE
+    if os.path.exists(REBOOT_CAUSE_FILE):
         os.remove(REBOOT_CAUSE_FILE)
 
     # Write a new default reboot cause file for the next reboot