enb,ue,process: move stdout counter helper to process objects

this patch adds the stdout counter to count events happening
on the stdout (known from the UE already) to the common
process class so they can also be used from the eNB (and other objects)

In addition, we add a PRACH counter to be used for tests.

Change-Id: I434f072b8aa6f4dce9f90889c6b40832f6798ff8
diff --git a/src/osmo_gsm_tester/obj/enb_srs.py b/src/osmo_gsm_tester/obj/enb_srs.py
index 6afc873..bf988eb 100644
--- a/src/osmo_gsm_tester/obj/enb_srs.py
+++ b/src/osmo_gsm_tester/obj/enb_srs.py
@@ -258,6 +258,11 @@
     def running(self):
         return not self.process.terminated()
 
+    def get_counter(self, counter_name):
+        if counter_name == 'prach_received':
+            return self.process.get_counter_stdout('RACH:')
+        raise log.Error('counter %s not implemented!' % counter_name)
+
     def get_rfemu(self, cell=0, dl=True):
         cell_list = self.gen_conf['enb'].get('cell_list', None)
         if cell_list is None or len(cell_list) < cell + 1: