Change to update ffdc_prefix only at pre_boot call point.

Change-Id: I30a9f8074a1c4ac78c131edb06afe8b5c45eb34b
Signed-off-by: Michael Walsh <micwalsh@us.ibm.com>
diff --git a/lib/obmc_boot_test.py b/lib/obmc_boot_test.py
index f407569..07e9f77 100755
--- a/lib/obmc_boot_test.py
+++ b/lib/obmc_boot_test.py
@@ -60,6 +60,7 @@
 boot_count = 0
 
 LOG_LEVEL = BuiltIn().get_variable_value("${LOG_LEVEL}")
+ffdc_prefix = ""
 
 
 ###############################################################################
@@ -252,12 +253,6 @@
     else:
         test_really_running = 0
 
-    seconds = time.time()
-    loc_time = time.localtime(seconds)
-    time_string = time.strftime("%y%m%d.%H%M%S.", loc_time)
-
-    ffdc_prefix = openbmc_nickname + "." + time_string
-
     BuiltIn().set_global_variable("${test_really_running}",
                                   test_really_running)
     BuiltIn().set_global_variable("${boot_type_desc}", next_boot)
@@ -311,6 +306,14 @@
     except OSError:
         pass
 
+    global ffdc_prefix
+
+    seconds = time.time()
+    loc_time = time.localtime(seconds)
+    time_string = time.strftime("%y%m%d.%H%M%S.", loc_time)
+
+    ffdc_prefix = openbmc_nickname + "." + time_string
+
 ###############################################################################